In my survival world, I created a simple passive mob pit. They sit on the trapdoor as I breed them, and once I got enough I would remove the dirt and open the trapdoor. I went into the nether, and when I came back I heard the sound of chickens dying and I found chicken remains about 10 blocks from the storage of my chickens. I had a yellow sheep that somehow was able to switch into the chicken holder. All the other sheep and chickens were gone. The cows and sheep were on open and closed trapdoors and there was just one of each left below the trapdoor. This also happened to me again when I reloaded the world.