Quote Originally Posted by Justiciar
If you're playing as Pahlava or the Koinon Hellenon.. Get used to it.

It's down to the faction cultures.. when a city rebels it goes to the faction with the closest culture. For many cities around the med it's KH, while in the Arabian peninsula they tend to go to Parthia.
Close.

There are two main things that affect a city revolting to a faction: the default_faction in descr_regions.txt, and the faction_creator in descr_strat.txt. The faction_creator determines things like the city's appearance, while the default_faction (I believe) is who the city would prefer to revolt to. I believe both have at least some influence over cities revolting, at least early in the game.

From what I've seen, the effects of these are less with the 1.5 patch, though I'm not 100% sure.