Hypothesis #2 Although the physical exam did not reveal any trauma, this does not rule it out. We have a large breed puppy, ,most likely very active. If the joints have not finished forming/closing the might predispose to a joint injury by unobvious means (pulled out of joint, torn ligaments in the hip, physeal fractures among others, even fracture of bones. ) just because there was not physical direct evidence of trauma does not mean there was not some type of disruption in the normal integrity of the musculoskeletal system. We also can not rule out yet possible trauma to the vertebrae/spinal column