Through the years, we have had other members who have worried over unexplained tremoring episodes. My own Cushpup, a Lab, suffered from intermittent involuntary tremoring episodes across his shoulder blades that made him look as though he was shivering. For him, the episodes happened most often when he was sitting down or at rest -- even when he was sleeping. Sometimes he would jerk himself awake, they were that severe. But just as is true for your boy, he would usually just stretch afterwards and never seemed to be in any particular pain or discomfort. The tremors began before he started trilostane treatment, worsened significantly when he first started the drug, and then ended up stopping entirely a few months down the road.
We never did figure out what caused them. Through the years, other members here have reported similar issues with their dogs under a variety of circumstances: some using trilostane, some using Lysodren (another medication), and also dogs not yet being treated at all. For this reason, it has been hard to point to a single "trigger." It is true that tremors are now listed as a possible side effect of trilostane. But I've always wondered whether such tremors more often just somehow relate to rapidly changing cortisol levels. But the good news is that they didn't really seem to bother my dog at all -- it was ME who was going crazy trying to figure them out!