The condition is known as "congenital vestibular disease" and unfortunately has been present in numerous litters of Sumatran tigers, both in Australia and Overseas. It can effect (to varying degrees - from the completely obvious to the so slight it is undetectable to the untrained eye) just individuals or entire litters of siblings. Symptoms, both physical and neurological, are much more prevalent in younger animals and seem to subside as the animals mature. There are various theories as to the cause of this disease, but I would agree that it could be a result of inbreeding, which of course with such a small founding population is almost unavoidable.