for what it's worth, I can abandon any city filled with celts and sumeria doesn't care.
abandon a roman or an arabian city (which are relatively nearby) and they are unhappy.
i wonder... did carthage have mapmaking when they bit the dust? Is it at all possible that they met the sumerians - the AI could have sent a galley across to the sumerians, and maybe once that contact is made, the citizens still send news of bad deeds around until they are completely subsumed?