Most people don't expect a horse to roll while mounted, so the first time it takes you totally by surprise. You know that the horse is doing something strange, but it doesn't dawn on you what until it's too late. Once you can recognize what they are starting to do, a good way to stop it (before it goes too far) is to *insist* on forward motion (kick the snot out of them). Of course, you should immediately try to figure out why they did it.
While out on a trail ride, some horses will instinctively try to roll when you cross them over water. It's not a good idea to leave a saddled horse untied as many will roll with the saddle on and break the tree. One horse we knew had developed the habit of rolling as soon as she was mounted, just to see who was in charge ;)