I’ve made a thread mentioning this bug:
To sum it all up: since one of the latest updates (I believe 2.3?..), all NPCs that get cornered or pushed back will “ghost” and get through things. In the case of archers and others that use long distance attack, they will get stuck in the mesh and shoot from there. If they are melee, they will just get stuck. Also, before this, if you pushed back an NPC against the edge of a cliff, they would resist being pushed.
In the case of archers, they will fall off when backing away to shoot. Now, they will, as you just wrote, plunge backwards. This looks specially stupid if they are your archers positioned atop a tower, and a purge ensues… Just watch in awe as they fall from their positions…
I believe that these changes were done in order to mitigate combat cheesing. But, coupled with the undermesh problem, it backfired badly… Nothing broke this game for me so much as this issue now…
If they are going to keep them, at least make it so that your enemies suffer fall damage when jumping backwards from a cliff! 