That is how things work in EB. As the Romanii, I still havent been messed into the syrian wars, but I have built a large network of spies. Recently while playing EB, I had a diplomat wondering about in Asia Minor. I noticed that the city of Ankyra! which is in AS hands had a 95% population happiness, but there where no characters. Here comes my best spy, he infiltrates the city and happiness drops to about 55%. The next turn the city begins to revolts, the population had enough of AS domination. The next turn the galations join the Avernii. This is a great way to weaken your enemies. To avoid revolts and unrest in your cities bordering other factions always leave a spy and a diplomat there, they will do the trick.

Cheers.