[Rumil]

I wonder if it was the presence of Gandalf, as well as the presence of the ring, which roused the Balrog to attack the Fellowship.

I imagine that maiar could sense one-anothers' presence and that the Balrog would have considered Gandalf a threat to 'his' domain, while Balin's Dwarven colonists provoked no such response as they were beings of little 'power' in the view of the Balrog.

Maybe!