A hypothesis from an old Timur Kuran paper:
A feature shared by certain major revolutions is that they were not anticipated. Here is an explanation, which hinges on the observation that people who come to dislike their government are apt to hide their desire for change as long as the opposition seems weak. Because of this preference falsification, a government that appears unshakeable might see its support crumble following a slight surge in the opposition's apparent size, caused by events insignificant in and of themselves. Unlikely though the revolution may have appeared in foresight, it will in hindsight appear inevitable because its occurrence exposes a panoply of previously hidden conflicts.
(Photo: An anti-government protester draped in an Egyptian flag stands near government loyalists down the block in Talaat Harb Square the afternoon of February 4, 2011 in Cairo, Egypt. By Chris Hondros/Getty Images. Hat tip: Cowen.)