Sometimes the animals won't move at all except for eat grass.

Sometimes when I'm playing Minecraft, all of a sudden a few animals or only one will just stop moving, except for maybe put their head down to eat grass sometimes. An example of this happening to me is a baby sheep jumped onto a hedge I made from leaves and it wouldn't move except for put it's head down to eat grass and when it lifts it's head back up a random block of grass change to dirt in the close area as if the sheep was somewhere else. I tried hitting the sheep but it didn't run. It stayed there until I forced it to disappear by walking to far away from it. FYI this isn't only with baby animals. It happened to me with two adult horses and one baby horse too.