I had 13 chicks all between 1 and 2 weeks old. They have all been doing great running around eating and drinking good. They also get along great and don't pick at each other. I did have to rehouse two bullies a few days ago, but everyone else has done great. They were starting to attempt escaping the brooder last knight so I put a frame made with wood and hardware cloth over it, and when I got up there was a little chick away from all the others flat on his back dead. All of them were fine last night, and the remaining chicks are doing great now. What could have happened?