it occurs randomly, shar teel initiates in party dialogue where she tells dorn that she has enough of him and want to fight with him
//well, since dorn is fully voiced in that dialogue, it seems that there is some bad relationship
It is a vanilla scripted fight. If there is a problem, you can advance progress with reading the value of
Global("DORN_SHARTEEL_FIGHT","GLOBAL")
and just incrementing it stepwise by 1 until you solved the issue.