Some mobs end up falling and stuff because of their AI. This includes mobs trying to walk on signs and open trapdoors, and stepping on pressure plates that cause the floor to give away. They also walk on dripleaf or whatever its called and they sometimes stop, so they fall.

How to reproduce:
Try anything I said above

What I expected:
The mobs will avoid falling and stuff

What I saw:
The scenarios I said above happen instead.