I was also thinking it is a pain response. But he might also be only able to tolerate small bouts of play before he feels overwhelmed. Merlin can be like that with other dogs, though not with Cadbury, but then I can tell when Cadbury is pushing him too far and make him stop.
I fully expect Merlin to become a grumpy old man who gets fed up being played with!
I remember a Leonberger that was dog friendly but had arthritis in its paws. It would play lovely until a dog came near its paws, even without touching, and it would become aggressive because it had associated pain with a dog being near its paws.