I hadn't noticed this for some time, but I saw it again last night.

Sometimes when teams are unbalanced, the auto-balance mod kicks in, as it should. Unfortunately, it sometimes switches two people over from one team to the other so it still remains unbalanced. If there are 9 players on BLU and 7 on RED, it sometimes switches two players from BLU over to RED so it remains 9 vs. 7 instead of 8 vs. 8. I believe it happens if the two players die at roughly the same time, but it's a bit counter-productive to have the game have to balance twice.