We had a beautiful Dorking cockerel that produced too many chicks with splayed legs so we switched to our much more ordinary reserve cockerel and the problem disappeared.
I'd always heard that breeders check the lining on the leg scales to see whether or not a bird was a breeding prospect. I got birds off breeders who didn't tell me why they got rid of them; I later found out. Chicks started hatching who were spraddled outways or inways; it was caused by an inherited deformity, a misalignment of bone, muscle and tendon, and it shows in the parents as incorrect leg scale lining. They walk fine, but their offspring will suffer, and only produce more of the same.