I actually have a neighbor that watched as her chickens came out of the coop one morning. Everything was normal, till one came flying out, scared another one that tried to run under the coop and it smacked it's head. When my neighbor walked over to see the commotion, the one that tried to run under the coop, was dead. She said if she hadn't been standing there, watching, she never would have known what happened. The dead girl had zero signs of trauma, just dead next to the coop. Anything can happen, it's part of the chickens being chickens thing.