I've had this often enough... probably like 8 or 9 times now, once it was even 2 people on a team. I always presumed its just those players either crashing, or not connecting to the match properly. The fact that they also have it cancel the match out and return everyone to the menu screen leads me to believe that that's just the solution to that problem. It's also better than going into a match down a player.
I don't believe this is a bug