My first thought was broken saddle tree. Glad to hear you were bareback.
I have a gelding that loves to try and roll. But I see the signs and just put him to work when he's under saddle. Whe we are coming down the trail and I see him putting his head down to look at a sandy spot, or in the winter, he'll start looking at fluffy powder snow.
Now once the the saddle is off, It's fair game for him to roll after a ride.
I always lead my horses over to a nice spot and give them the chance before I load them up in the trailer.
Or a video of him rolling in the sand.