Hm, questions questions, questions. Not sure bro, but I'd say that its programmed to become your enemy if it is attacked by other families of the Romans, but then it may not be programmed to stay your enemies.

My experience was, that when some of the Romans enemied me, the others didnt, and in other Campaigns, when some of them enemied me, say Bruti, but not Scippi or Juli, they all enemied me. Just random selection possibly?