Hypothesis 1
The tendon gaps indicate a failure in the tendon which allows the gastroc muscles to be flaccid and the hock cannot be supported in extension so the dog walks "down" on the back of the tarsus. One side failed because it was weakened or traumatized which put strain on the other side and it failed because of overwork. The lack of other signs, especially neurological support a local failure rather than central or generalized. The activity level in an aging dog which has been on a sub-par diet may contribute to the weakness that led to failure. Age may have been a factor. Overwork may also have played a role. Sub-clinical muscle inflammation / lactic acid buildup may have predisposed to muscle deterioation and failure.

Hypothesis 2
Similar to 1 except the failure is in the tibiotarsal bone. The gaps palpated are a contracture of the muscle / tendon which has pulled the bone fragment up and away from the rest of the bone and left a gap. All the remaining rationale in 1 apply here as well with the emphasis on bone instead of tendon / ligament.

Hypothesis 3
A tumor or degenerative process has affected the nerve supply to the hock extensor mechanism. The motor nerves are affected and the motor function of the stifle and hip joints are not affected. Neurological function - pain sensation, proprioception, motor function are all working except for the motor to the hock extensors. The gaps that are palpated are because the muscle is flaccid. The lesion is in the ventral roots of the spinal cord at the level(s) at which the hock extensor nerves come out of the canal or the cord.