I've only seen this once, but it might be worth looking into to figure out why it happened. In short, the matchmaking algorithm put 26 players into the match. Among the players was at least one party of eight. So far, so good.
But then, it divided up the teams like this:
The teams stayed like this through the match (with the usual, occasional player joining and leaving). Not sure how matchmaking came up with 21-versus-5, but that probably shouldn't ever happen.