I have a four blocks wide tunnel going downwards in my pond for axolotls.  The axolotls would only stay inside that tunnel and wont swim anywhere else.  Even if I place an axolotl far away from the tunnel, soon it would swim towards it and get stuck.

I have also spot a similar behavior on axolotls that spawned in the swamp.  Several axolotls were stuck and cramped inside a 1block-deep hole that were 4~5 blocks wide.

This kind of behavior was spotted on a multiplayer server, although I'm not sure if the same thing occurs on single-player worlds.

-update

i blocked the tunnel aaaaand the axolotls are all gone except for one..