peaceful mobs randomly begin to spin on top of a fence

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. make a 5x5 fence area
2. put a peaceful mob (sheep,cow horse pig,chiken) and put a block next to the fence
3. wait for the mob to stand on the fence and....

it becomes a tornado