Angulation is a very plausible cause (or a genetic predisposition). That might be supported by the higher incidence in spayed/neutered dogs- if the neuter was done before maturity (I don’t know if there is any data differentiating between early and later spay/neuter on this). Without normal hormones the growth plates in the bones close late, so they get longer than they were genetically programmed to be. The angles of the joints are not what they would have been and the soft tissues don’t change to compensate for the longer bones. The angulation could also be really thrown off if the hormones are removed when some growth plates have closed, but others haven’t. That’s one theory I’ve read anyway. Another is that neutered dogs are at a higher risk for obesity, which is in turn associated with cruciate ligament injuries. Is it possible that lack of hormones causes weaker soft tissues too? Only guessing that could be a possibility.
There is no good study I’m aware of, but ancedotally there is apparently some evidence that the incidence of hip dysplasia is higher in early spayed/neutered dogs.
Lisa