If he knows how to sit and won't sit for you, and also won't sit on his own, take him to the vet and get him checked for something that might be giving him pain. I don't think the crating would do it unless he got stuck in some awkward way and sprained his back, but anything can happen to a dog being boarded and they aren't going to risk their livelihood by telling you what did happen. He might have a sore anus or blocked anal glands, a cramping or blocked intestine, or something that hurts him about his muscles or back. See the vet.