Revolutions have been, and will remain, facts of life because of the structural nature of prevailing relations of production and relations of political power. Precisely because such relations are structural, because they do not just “fade away” - as well as because ruling classes resist the gradual elimination of these relations to the very end-revolutions emerge as the means whereby the overthrow of these relations is realized.
From the nature of revolutions as a sudden, radical overthrow of prevailing social and (or) political structures-leaps in the historical process-one should not draw the conclusion that an impenetrable Chinese wall separates evolution (or reforms) from revolution. Quantitative gradual social changes of course do occur in history, as do qualitative revolutionary ones. Very often the former prepare the latter especially in epochs of decay of a given mode of production. Prevailing economic and political power relations can be eroded, undermined, increasingly challenged or can even be slowly disintegrated, by new relations of production and the political strength of revolutionary classes (or major class fractions) rising in their midst. This is what generally characterizes periods of pre-revolutionary crises. But erosion and decay of a given social and/or political order remains basically different from its overthrow. Evolution is not identical with revolution. One transforms dialectics into sophism when, from the fact that there is no rigid absolute distinction between evolution and revolution, one draws the conclusion that there is no basic difference between them at all.
The sudden overthrow of ruling structures is, however, only one key characteristic of that social phenomenon. The other one is their overthrow through huge popular mobilization, through the sudden massive active intervention of large masses of ordinary people in political life and political struggle. 
One of the great mysteries of class society, based upon exploitation and oppression of the mass of direct producers by relatively small minorities, is why that mass in “normal” times by and large tolerates these conditions, be it with all kinds of periodic but limited reactions. Historical materialism tries, not without success, to explain that mystery. The explanation is many-dimensional, drawing upon a combination of economic compulsion, ideological manipulation, cultural socialization, political-juridical repression (including occasionally violence), psychological processes (interiorization, identification), etc.
Generally, as one revolutionary newspaper wrote at the beginning of the French revolution of 1789, oppressed people feel weak before their oppressors in spite of their numerical superiority, because they are on their knees.  A revolution can occur precisely when that feeling of weakness and helplessness is overcome, when the mass of the people suddenly thinks “We don’t take it any longer,” and acts accordingly. In his interesting book, The Social Bases of Obedience and Revolt, Barrington Moore has tried to prove that suffering and consciousness of injustice are not sufficient to induce large-scale revolts (revolutions) in broader masses. In his opinion, a decisive role is played by the conviction that suffered injustice is neither inevitable nor a “lesser evil,” i.e. that a better social set-up could be realized.  A concomitant brake upon direct challenges to a given social and/or political order, however, is the locally or regionally fragmented nature of revolts pure and simple. Revolts generally become revolutions when they are unified nation-wide.
Such challenges can be explained, among other things, by that basic truth about class societies formulated by Abraham Lincoln, empirically confirmed throughout history, and which is at least one reason for historical optimism (belief in the possibility of human progress) when all is said and done: “You can fool all of the people some of the time and some of the people all of the time. But you can’t fool all of the people all of the time.”
When the majority of the people refuse to be fooled and intimidated any longer; when they refuse to stay on their knees; when they recognize the fundamental weakness of their oppressors, they can become transformed overnight from seemingly meek, subdued and helpless sheep into mighty lions. They strike, congregate, organize and especially demonstrate in the streets in increasing numbers, even in the face of massive, gruesome, bloody repression by the rulers, who still have a powerful armed apparatus at their disposal. They often show unheard of forms of heroism, self-sacrifice, obstinate endurance.  This may end in their getting the better of the repressive apparatus which starts to disintegrate. The first victory of every revolution is precisely such a disintegration. Its final victory calls for the substitution of the armed power of the revolutionary class (or of a major class fraction) to that of the former rulers. 
Such a descriptive definition of revolutions has to be integrated into an analytical-casual one. Social revolutions occur when prevailing relations of production cannot contain any more the development of the productive forces, when they increasingly act as fetters upon them, when they cause a cancerous growth of destructiveness accompanying that development. Political revolutions occur when prevailing relations of political power (forms of state power) have likewise become fetters upon a further development of the productive forces within the framework of the prevailing relations of production, a development which is however still historically possible. That is why they generally consolidate a given social order, instead of undermining it.
This materialist explanation of revolutions offered by Marxism seems indispensable for answering the question: “why, and why just at the moment?” Revolutions have occurred in all types of class societies but not in a uniform way. It appears clearly illogical to attribute them either to permanently operating psychological factors (humanity’s allegedly inborn aggression, “destructiveness,” “envy,” “greed” or “stupidity”) or to accidental quirks of the political power structure: particularly inept, stupid, blind rulers, meeting increasingly self-confident and active opponents. According to the particular school of history concerned, one can see that blind ineptitude either in the excessive recourse to repression, or in the excessive amplitude of suddenly introduced reforms, or in a peculiar explosive combination of both. 
There are of course kernels of partial truth in such psychological and political analyses. But they cannot explain in a satisfying way the regular and discontinuous occurrence of revolutions, their cyclical nature so to speak. Why do “inept” rulers at regular intervals succeed “adequate” ones, so many times in so many countries? This can surely not be caused by some mysterious genetical mutation cycle. The big advantage of the materialist interpretation of history is to explain that occurrence by deeper socio-economic causes. It is not the ineptness of the rulers which produces the pre-revolutionary crisis. It is the paralysis engendered by an underlying social-structural crisis which makes rulers increasingly inept. In that sense Trotsky was absolutely right when he stressed that “revolutions are nothing but the final blow and coup de grâce given to a paralytic.”
Lenin summarized the underlying analysis in a classical way by stating that revolutions occur when those below do not accept any longer as before. The inability of a ruling class or major fractions to continue to rule has basically objective causes. These reflect themselves in increasingly paralyzing internal divisions among the rulers, especially around the question about how to get out of the mess visible to the naked eye. It intertwines with growing self-doubt, a loss of faith in its own future, an irrational search for peculiar culprits (“conspiracy theories”) substituting for a realistic objective analysis of social contradictions. It is this combination which precisely produces political ineptitude and counterproductive actions and reactions, if not sheer passivity. The basic cause always remains the rotting away of the system, not the peculiar psychology of a group of rulers.
One has obviously to distinguish the basic historical causes of revolutions from the factors (events) triggering them off. The first ones are structural, the second ones conjunctural.  But it is important to emphasize that even as regards the structural causes, the Marxist explication of revolutions is by no means monocausally “economistic.” The conflict between the productive forces and the prevailing relations of production and/or political power relations isn’t all purely economic. It is basically socio-economic. It involves all main spheres of social relations. It even eventually finds its concentrated expression in the political and not in the economic sphere. The refusal of soldiers to shoot at demonstrators is a political-moral and not an economic act. It is only by digging farther below the surface of that refusal that one discovers its material roots. These roots don’t transform the political-moral decision into a pure “appearance,” or a manifestation of mere shadow boxing. It has a clear reality of its own. But that substantial reality in its turn doesn’t make the digging for the deeper material roots irrelevant, an exercise in “dogmatism” or an “abstract” analysis of only secondary interest. 
In any case, the inability of the rulers to continue to rule is not only a socio-political fact, with its inevitable concomitant of an ideological moral-crisis (a crisis of the prevailing “social values system”). It has also a precise technical-material aspect. To rule also means to control a material network of communications and a centralized repressive apparatus. When that network breaks down, the rule collapses in the immediate sense of the word.  We must never, therefore, underestimate the technical aspect of successful revolutions. But the Marxist theory of revolution also supersedes a peculiar variant of the conspiracy theory of history, which tends to substitute for an explanation of victorious revolutions an exclusive reference to the technical mechanism of successful insurrections or coups d’état.  Instead, it is the material interests of key social forces and their self-perception which provide the basic explanation of turning points of history.