Upon loading a world that has multiple entities confined in a small space or up against a wall while affected by a water current especially a 1x1 area, the entities will phase through the terrain, escaping captivity and creating a huge mess. They do not seem to fall through floors, but will go straight through multiple-block-thick walls. The force that propels them seems to be mob overlap repulsion, but it can also occur with water, allowing this bug to affect a single mob with nothing (but walls) nearby.
If I had to guess, the issue is that mobs begin moving before terrain physics are loaded properly - when you load into a world, you can catch a solid five seconds in which chickens flap their wings as if they were airborne. For some reason, this does not affect the player at all.