I just had a long string of games in a row where 2 or more players on the other team quit at the start of the match. This "opt out" matchmaking seems to be to blame. People have no intention of playing a new game, but are busy admiring their stats and are inadvertently rolled into the next game and have to abandon the match in order to quit playing.

It wouldn't be so bad if matchmaking made any effort at all to replace them, but it never has and still doesn't. What a shame.