Revolution, like many social changes, often is a contagion. Had the Americans not revolted against the British, the French would not have revolted. Had the French not revolted, Haiti may not have and the independences movement continues to move across the world in waves. If enough people are unhappy with a ruling system and enough support comes behind a new system, then uprising happen. Add military, funding, foreign aid and support, with a few strong-willed rulers and you have a revolt. Sometimes the aftermath brings much needed changes, sometimes it is the start to decades of unrest.