It sounds like it probably had some kind of neurological problem, maybe a stroke or a seizure. I would feed him off, because you're not going to want to breed from him.
Something else it could be, is malnutrition. Was the animal thin looking, with a bony tail and a 'big' looking head? This would point to malnutrition as a culprit.
Also check the skull for deformities. If the forehead looks very bulbous, it could have a deformity that causes physical manifestations.