My first horse, many yrs ago, loved to roll. He would roll everytime we turned him out to the pasture after we rode. It wasn't unheard of him to try and roll with me & saddle on. But it would only happen when we were riding and had to stop and wait for other horses ahead of us to start going again. He wasn't always particular in what he rolled either; snow, water, sand, newly tilled field. I do think part of the problem may have been dry skin because of all the rolling and scratching on our fence posts he would do. He also loved it when we curry combed him.
My recommendations would be:
1.see how dry his skin is.
2.The vet may think he physically fit, but a chiropractor may be able to give a different perspective.
3.The more riding you do with him, the more you'll be able to see the little hints. It took me a bit. As soon as my horse put his head down to smell the dirt/snow/etc, I knew the next step was going to be his front hoof do one paw mark. This would take 1-3 seconds. As soon as I suspected he was even thinking about it, I got him moving.
4. I would also check the saddle fit, the girth and the blanket. Make sure there's no pinch or any burs hidden.
As a side note, my horse was a little crazy arabian that I loved and learned a lot on. But I have never seen another horse have a personality & attitude like him. We had a love/hate relationship.