OP, this is happening to me as well as my GM, both mages.
Even when I’m incredibly careful not to do anything to put me on any other mob’s aggro tables, I still will get attacked by the other player’s mob if it has been deleashed or they died. The worst part is that it will still be tagged by them, so I can’t polymorph it.
The follow up posters are correct in that healing, buffing, or AoEing will put you on an aggro table, however in my case I have been able to replicate this behavior when I was specifically making sure I did nothing to generate aggro with any other mobs.