Who knows....I don't think it was weight thing at least for the Isa's .They seem smaller than the others but laid like clock work. I guess this is my Welcome to the real world of things that can happen when having chickens.Very Cute!
I had that happen with two of mine last spring. They looked the same as yours. It happened out of the blue no signs or warning. My only conclusion was they had gained too much weight during the winter with all the treats and extra protein I gave them. I don't know if that is right but it was the only thing I could come up with that I thought might have caused it.