That is truly a bummer! I'm sorry for your loss! What are the sides of your brooder made of? My girls are 4-5 weeks old and they are starting to get a little crazy too. I get worried they will try to fly over the brooder edge and hurt themselves, but you can't control everything I guess!
Chicks are in the top part, rabbits live in the bottom portion. Sides are hardware cloth, with a few wood support pieces. The chick most likely crashed into a wood support and broke it's neck. Death was very quick. And there is absolutely nothing that could have prevented it. My brooder set up is about as safe as it can possibly be. Chicks just have a way of finding new ways to kill themselves. At least it wasn't one that I was planning on keeping anyways. If it was a pullet, she would have been sold. If a cockerel, would have ended in the freezer.