Probably a broken leg due to weak bones. When I butcher mine I catch them by the legs. I had several snap when picked up by the legs. The leg turns all different colors of green, purple, red immediately and has to be thrown away.
If you had 11 pound chickens at butcher weight (which is just a few weeks) then it's no wonder their legs were snapping. Their bones are still pretty soft because they're so young, and all that weight will dislocate their joints picking them up like that. They usually don't move fast enough to catch them by the legs when they're that big. Maybe just pick them up the next time - it'll save them some pain and save you some meat.