Neither of my horses have ever rolled with me on them, however I have been with a friend when hers "attempted" to roll. Was a hot day, we were crossing in a field, tall grass, wind whipping across, horse was probably itchy from drying sweat, well you get the picture.
Another incident happened when we were crossing deep "puddles" on the trail. Again , hot day, and I'm sure it seemed like a good idea at the time to the horse. Not the same horse or rider.
They will usually give you an indication that they are going to go down, just have to pay attention and get them moving. Of course, I'm sure there are exceptions, usually are.