The thing is, there was absolutely no reason to do that even from a "Secure the Boss at all costs" viewpoint.
If Touma had cut the bridge himself when they had finished crossing, the monsters would have fallen off then.
And if some monsters had climbed up the cut bridge, they would easily have been dispatched, since the issue wasn't their strenght, but their numbers. Hell, even fighting on the other side of the bridge would have been easy.
These guys just threw their jobs away (and probably earned a beating too) for no good reason.