I know it is caused from thin yolk walls, but I suspect the reason for the thin wall could be one or several factors involved, including the health of the hen.
All my searching points to aging of eggs. Even in official documents.
A fresh egg will have a round yolk inside a thick egg white, encircled a thinner egg white.
An egg which has had time for air to enter the shell will have a flat yolk, and watery egg whites. The yolks walls will be so...
WHAT? No one has an answer. My chickens are now doing this. Some have been laying almost a year now, and others have just begun. I'm going to do a search.