LotR -- Book 2 - Chapter 06 - Lothlórien

[Legate of Amon Lanc]

Oh, I see. But then, to settle the matter, I think the answer is in the book. Gandalf says:

Originally Posted by The Bridge of Khazad-Dum

If we had come by the main road down from the upper halls, we should have been trapped here

And if you remember, the Chamber of Mazarbul was one of the side chambers of the large hall to which the Fellowship arrived, and to which they intended to return after they spent enough time at Balin's tomb. So, the Orcs' original idea was: light a fire by the main route (assuming, quite correctly, that the Fellowship would use the main route) and attack the company from behind, so that they cannot escape. By taking a turn and leaving the main route for a short while, it happened that when the Orcs arrived, the Fellowship was off-course for the planned trap. It could have still gone well for the Orcs, had they allowed the Fellowship to return to the main hall - which, actually, not to be too unjust to Orcs, they could have done; but it was the Fellowship's choice to block the door and defend themselves inside the room. This way, the Orcs had to attack them in the room, thus discarding the possibility to catch the Companions in the fire. They still had a good chance in at least pursuing the Fellowship (the Orcs still were in larger numbers), however the collapse of the chamber after Gandalf's spell duel with the Balrog forced them to use the route they themselves trapped before.

So, hope this explains it, and if so, settles the question