Is your dog double jointed? Sitting to one side can often be mistaken for being 'double jointed', as Roberta Baxter explains...
(Q) My dog rarely sits straight down on to his hindquarters, he usually sits to one side. Someone said he was double jointed - is this right?
(A) Vet Roberta Baxter says: Double jointedness is not normally an issue in dogs, however dogs often sit to one side if they have back, hip, or stifle pain on one side. I would be concerned that your dog might be stiff or painful, which might be causing him to sit asymmetrically. Other signs that this might be the case would be stiffness after rising from rest, and bunny hopping when moving at speed, or even overt lameness.
It would be a good idea to get your vet to check your dog out and see if he does have a problem - if so, anti-inflammatory medication might be helpful, and your vet could advise you on other treatments.