If you're playing as a hostile mob and attack other mobs and then leave your current body, the mob you were possessing will kill itself.

Note: Pretty sure your mob will also kill itself if you've alerted other hostile mobs nearby by touching/getting touched one of them if i remember correctly.

this also happens on old versions just to pyro know

this still happens in 1.18.2 even without alerting other mobs. when you leave a mob it sometimes just attacks itself and this is fixed by enteing the mob again