[inglor_cz]

I think any explanation should also explain all the instances of countries where protests did not happen, even though inequality was high.

The best evidence for causality would be to demonstrate some kind of "dose and response effect", which isn't easy. The dreaded word "culture" tends to creep in, because some countries seem to have a lot more riot-y history than others. (France vs. UK vs. Spain).

[Mikeb85]

You're right, social mobility and overall standard of living matters as well. Also, how strong the state itself is and how decisively they can put down insurrection...