That mat in your brooder could be an issue. Their tiny toes may be getting tangled in the floor mat and in their panic to free themselves, they may be causing themselves injury. Wire flooring caused issues with my quail chicks as they spend a lot of time just eating or dozing, and even with a quality hardware cloth floor, they'd get tangled up in it. I've switched to cheap terry cloth bath towels that I fold to size and swap out daily. With a quick shake outdoors to dislodge the clumpy bits, they're easy to launder.