Unless you have done a whole lot of necropsies, I would recomend having your veterinarian do it & send samples into a lab for pathology if you really want to know why it happened. It doesn't seem to be anything contagious, because no one else is showing anything unusual.
Thank you for such a nice welcome! Most people think chickens are chickens are chickens. They each have different personalities & are so fun to watch & interact with. I really miss them. but I think this site will help!
I'm a retired veterinarian who raised & enjoyed chickens my entire adult life. Unfortunately, I've got MS, am now in a wheelchair & can't work or raise chickens anymore. So I will enjoy them through all of your eyes & comments. If I can help answer any medical questions about them, I will be...