This, to me, isn't really that big a deal. I personally think I was being tested here but tell me what you think. I rode my horse for about 20 minutes on monday. As soon as we got out there not even one lap around the arena, she stopped. I was actually testing her to see what she would do, such as roll. She has never rolled before with anyone on her. She dropped down and rolled. I was able to get off in time though. Also, she hasn't really been rode all winter long, just let out to frolic here and there. Still, no excuse though and I'm aware of that.
When she did, I stood there and waited for her to stop. As soon as she stopped and stood up, I took the reins off and ran her around the arena for a bit, both ways. Than stopped and got back on her. She was fine after that. I rode her yesterday and she didn't even attempt to roll. Why do you think she did this??