juliet

It's prices. You get forced revolutions when a critical mass of people can't afford food, or when the average person's standard of living falls (or increases slowly enough in a freeish media context that they notice they are doing much worse than comparable foreigners) consistently for a number of years in a row. We were heading into revolution territory in a number of countries over Summer, but the price of stuff has calmed down somewhat now.

Rallying points for the above might be ideological, but are just as likely in many societies to be ethnic. Autocracies can collapse more organically, because they are like companies rather than Governments, so 'failure' is a more likely outcome, and more likely to spread through the system if it is total in an individual part. Nonetheless, for the collapse of the Soviet Union I'd look at factors like oil production, oil prices (internal and external), and ethnic nationalists in the non-Russian SSRs.

In short, don't read political theory, read economics.