Because you were all in the right MMR band to be placed in a match together in the first place,
and you're all queuing back up at essentially the exact same time because you just got out of the same match?
You're inadvertently using the same trick groups of friends use to try to get into the same matches.

If it matters enough to you, a good rule of thumb is to queue back up immediately if it was a good close game,
and to wait 2-3 minutes to re-queue if it was a stomp in either direction, especially if there was someone trolling on either team.