When mobs are in moving minecarts, whether on rails or being pushed around off of rails, they sometimes start shaking/vibrating. About 1/4 of spawned mobs shaked when this happened. When I relog, it is not necessarily the same mobs that have this issue, but still about 1/4 of the mobs have this issue. While this attached video displays mobs spawned in with eggs, I originally discovered this in naturally spawn mobs. To recreate, just spawn some mobs, push them into minecarts, and move the minecarts around, and eventually some will have this very violent vibrating.