Ah. Yes. I've noticed that too-- highly annoying at times. Other than the tale of the gold-chevroned peasants of Noricum that I like to repeat so much, I also had Dumatha revolt to Parthia once, playing as the Egyptians. I've noticed that if this happens, the faction it revolts to goes to war with you. Does that happen with everyone? It was quite annoying, because I was going to suggest an alliance against the Seleucids with Parthia when they went to war with me over some grimy desert town.