Why are participants being brought back to the lobby after breakout room?

I was delivering a training session on teams. I put participants into breakout rooms for an activity. After the 5 minutes were up, instead of all being brought back to the main room, they were all taken back to the lobby? Can anyone tell me why, and how I can a) stop this from happening before it does b) deal with it  and stop it if it happens during the meeting

Thanks in anticipation!