A hot/sweaty horse and a pond of water .... or soft dirt .....
A lot of horses will try to roll because that's what they do when they are hot and itchy. They roll. I've had numerous horses over the years try to roll while riding (either in water, or in a dirt field). Basically start kicking and screaming to keep them on their feet and move along. It's a good way to break a saddle if they roll over on the top of it (much less the rider if the rider doesn't get themselves of the way if the horse goes down).
This is what the BO said, and honestly thinking about it now, I am CERTAIN this is what happened. Luckily I got off in time.