tail eating

a friend of mine got a hatch cock and hen from me, it bred well and he hatched alot of pure out of the pair.
this moult, he started eating his tail out, oil gland and feathers, chasing it then eating it. all of his sons are doing the same thing,,,wtf could cause this?
any idea what the hell would make this happen, the friend thinks the cock is retarded, but all the sons too???