A good rule of thumb is, if its not on your aggro list, combat assist wont attack it. This is so you dont just start attacking stuff when you target them while out of combat.
In a boss room and certain other places, the game automatically places you in combat when someone pulls, to avoid weird possible exploits when some of the party is fighting and the rest arent in combat. Since you're in combat, combat assist will start going on its own without you having to make the first attack.