It sounds a lot like heart failure to me and the one who died in the cone would have been because he couldn't breathe after he was turned upside down.
However, there are also a lot of chicken respiratory diseases. Chickens are very susceptible to respiratory issues. All breeds of chicken, not just Cornish Cross. Keep a close eye on the rest of them in case your flock has some sort of contagious sniffle.
However, there are also a lot of chicken respiratory diseases. Chickens are very susceptible to respiratory issues. All breeds of chicken, not just Cornish Cross. Keep a close eye on the rest of them in case your flock has some sort of contagious sniffle.