Not inbreeding. Happened to totally unrelated birds. Nothing that I read about it is consistent.Maybe it's due to inbreeding which can "fix" genetic traits both good and bad. Maybe it's because the goslings are weak or deficient in some way that we can't see. Or maybe it's a recessive or sex/linked trait like color blindness in men. But the fact that it never happens to whole sets makes me curious as to why it happens at all.