If you are queuing with a friend, then you increase your chances of a mismatch.
if your team contains any premade groups, then you increase your chances of a mismatch (even if you yourself entered solo).
If a player with high MMR (in Ranked) starts to play norms after a 3 month hiatus, then he'll wind up in low MMR norms, and it may take 100 matches for MMR to catch up.
If 4 Platinum players include a new player on their team, then they might be matched against a team of all Bronze. The chances of this happening is much higher in Blind, however.