Chickens are so interesting! I Didn't know any of thatA flock of hens always select just one or two nests as their favorite. I've even seen 2 and 3 hens in the same nest at the same time.
Creatures of habit.
It must have been a wind egg. Perhaps a piece of tissue sloughed off entering the oviduct which started the process of adding albumen and an egg shell with no yolk.