That's strange. I've only had bees for one year, so I'm certainly no expert, but I've never heard of them doing this. Not with healthy bees. So these were young bees as in still white??
The only thing that comes to mind (using my imagination here) is that maybe they had built some unstable comb on the bottom of a frame, and the queen laid eggs in it. Then when the brood grew, it broke off, and the bees were just doing some house cleaning. I know I often see clumps of comb on the bottoms of the frame. And the queen will lay eggs in there. When I catch it, I scrap it off on the landing board so the nurse bees can get back inside easily. If there's larvae in there, they don't like to leave it. It's sad to watch since the nurse bees look sort of frantic, as if they want to get the babies and help them. But I don't like them to build up the comb on the bottom of the frame since eventually it'll be a mess.