When a mob walks around aimlessly, it does this by choosing a random nearby location to walk to, then walks towards it until it gets there. If that mob is moved before it reaches its destination, it will continue to try to get there, even if that location is in an unloaded chunk. This occurs with teleport commands, riding mountable mobs, water streams, and any other way of moving them.

A related problem occurs with passive mobs and their "panic running" when they take damage. The panic run does not use this method, so a mob interrupted during their panic will finish their panic running and then stop normally. If however, they were already moving when they took damage, then this bug will occur and they will continue to move at increased speed until they get back to their original destination.