Could be a deformity. With meat chickens being bred so fast and so largely it might be something that got mixed up in the genes, and a lot of chickens are cross bred with family members (brother/sisters) because in the chicken meat farms, they don't care about anything of that sort just as long as the chickens produce meat. I would say that also when chicken houses (for meat) they feed the chickens all sorts of steroids and other things as well. Hard to say really but most likely something they were born with. I have seen a lot of strange things in chicken houses (meat chickens). I have seen chickens with three legs, chickens with two beaks, and all kinds of deformities.