Alright, I was playing as Byzantium, and before the current war I'm in had around 20 agressive expansion with genoa and savoy, and 15 with austria. Commonwealth called me into a war and I accepted, and genoa and savoy both joined. After I sieged them down, commonwealth gave me half of both of them and now everyone is joining a coaltion against me. It's not like I had the provinces set as vital interest either, the AI just decided to give me all this land and caused a coalation against me. This seems like a a big oversight, that an AI ally can just not understand AE and just cause a coaltion of everyone to form against you.