Comment 1 for bug 758112

I'd guess this is because of lag (seriously). IIRC the team for autoselect is chosen on the client side, and if someone takes the free spot on that team before the server gets your request, you won't be put on any team and remain spectator.

I'm not saying that this is good behaviour though, perhaps we should make autoselect a new option for the PLAYER_CHANGETEAM message to the server so that the team can be decided there.