I’ve heard and read that the normal chicken feed was the problem. Purina makes a brand that’s extremely widely used, Producer’s Pride. At some point last year the formulation changed and they didn’t tell anyone. Chicken egg laying went way down and in some cases even stopped. Once the egg farmers switched feed, the chickens suddenly started laying again like crazy.
https://typeset.io/papers/chicken-egg-yolk-antibodies-igys-block-the-binding-of-4k1kl8y6tr