This is a bug that seems to be random on who it affects, though it seems that the same players are (almost) always affected or not-affected.
Anyways, at a tag game, when the game is over for any reason, some players become stuck in their current spot, except for the house owner (one exception comes to mind for this) and some other players. Starting a new game 'fixes it' until the game is ended again.
The players that it does or doesn't affect seems to be random. Being friends with someone doesn't seem to matter, as some friends are affected while some non-friends aren't (and vice-versa). Doesn't seem to be related to the age of the account either, nor the players level, school, etc. Also doesn't seem to matter how the player got to the game (porting to friend, castle tours, etc).
I think it may be affecting other games too, but I'm not certain of that.