Revolutions occur when the masses understand that the system they live under is not an eternal structure handed down by God (or Cleon or whatever) but a Human-made structure that serves the interests of one group or another. Most peasants and city-dwellers in France probably thought for centuries that the semi-feudal absolute monarchy was eternal, right up to the point when the circumstances (especially the economic ones) caused the masses to realize that the structure was nothing more than the rule of the few over the many under various disguises, and hence the 1789 Revolution. Sure, the elite rules the opinions of the masses under most circumstances - but at certain points in history the contrast between these opinions and the reality reaches such a high peak that causes these opinions to shatter and be replaced, for a time, by the interests of the masses (more or less); even the worst police state (except for the Zhodani who REALLY control the thoughts of their Proles) cannot prevent this from happening - even East Germany, with about ONE THIRD of its population employed as agents of the Shtazi (state security police), reached the point in 1989 when the masses went out and smashed the Berlin Wall, state spies and rotten bureaucrats be damned. The British Empire, one of the greatest military powers in the 18th century, was forced out of North America by the colonial revolt of 1776 that gave rise to the USA.
For the most of history, the masses are a sleeping dragon. But when that dragon awakens, stimulated by the events of the era, no chains and no walls can hold it. This is why most governments, from the worst dictatorship to the some most enlightened democracies, invest so much in propaganda, in police forces, even in welfare (keep the masses happy and the likelihood of a revolt falls dramatically - see the Roman Empire's tactic of "Bread and Circuses").
And reforms might trigger revolution far more effectively than means of repression - as, in certain cases, they demonstrate the fact that the sate machine is not an eternal structure but a man-modifyable one. Give the masses a finger, under the right circumstances, and they'll want the whole hand - as they'll understand that the clenched fist is not the product of god but the result of the actions of man. Under Stalinism, Eastern Europe was full of repressive, jack-booted police states, but open revolts were rare (Prague and a few others). However, when Gurbachev, with his reforms, showed that the state machine could be changed for the betterment of the masses, everything fell apart - both due to the fact that the state displayed a clear inability to repress the masses, and because change was displayed as possible. Sure, there wasn't a popular rebellion in Russia in 1989, but there were uprisings and mass revolts all over Eastern Europe, resulting in regime changes and, in one case (Romania), the execution of the former tyrant. Unfortunately, the unorganized nature of these revolts gave rise to various opportunist elements, mostly within the former bureaucracy, who sought to plunder the remnants of the old economy; and to various new tyrants who saw an opportunity to start empire-building over the ashes of the old empire (Milosevic [sp?] in former Yugoslavia, for example).
