This happenes a lot, they stand in the battering ram, trying to find a slot, attacking if they can reach you. The very same thing happens with the cart on Hunger in the Dark.
The CW just came through the door which will break befor the escape. usually 1 CW stands in that part when we arrive to destroy the buboes, 1 stormvermin at the top, and specials drop from the top as well. And to be sure we saw the CW coming out through the door.