Animals die when they fall on slime blocks.

I have pigs hanging from fences so the pigs don't hit the ground, but I have slime blocks nearby and if they land on the slime block, they die. I tested it with cows too, same thing. It's only when they're hanging from a fence though - When I try to drop them from a high spot with slime blocks below, they live. Not sure if it's supposed to be that way or not.