Why does it seem that revolutions break out in waves (eg. 2011, 1989, 1968, 1848)?

When revolution was at in the Middle East and North America last year, it wasn't hard at all to find commentators compare the Arab Spring to the EU austerity riots or the Occupy movement, or even historical revolutions like Europe's Revolutions of 1848. If you want to be even broader, the American, French, Haitian, and Latin American Revolutions happen in pretty quick succession, and so do the Turkish, Persian, Russian, and Chinese revolutions at the beginning of the 20th century. What is it that makes so many people stand up against their governments at once? Does it have to do with global economic conditions? War? Cultural diffusion? Domino effect? Just people seeing patterns because they want to?