Charisma and random chance have nothing to do with it, but it's very deterministic.

Mobs keep track of the last person who hurt them, enabling them to track and all. So, if the tank was the *last person that round* to hurt the mob, it won't attack anyone; if a tick passes without another round (or an attack being used on it), it'll track the ex-tank, leaving combat unexpectedly... assuming the mob is a tracking type. However, if a damaging move IS used, the person using it will become the next tank, or if a new melee round happens... the first attacker is the tank. And melee round attacks always go in a determined order, from "newest creature" to "oldest", conjie mobs generally being newer than players (their position on the global creature list is higher).

I hope that made sense.