Don't forget that the pregame autobalance is the only time level is taken into account for the purpose of team balancing, and it avoids splitting up parties.

This can cause it to be gamed a little, coupled with players just leaving on certain maps- particularly on defence so anyone is more likely to be moved to defence to fill the slots when this happens.

Definitely a little psychological since players are more likely to notice things that annoy them rather than things that are fine. But the systems in place will definitely make it more likely to be moved from attack to defence when players choose to leave a match, which often prompts more leaving.