Longtime (45 years +) horse owner here, and very active trail rider and horse camper.
There are so many things that could have gone wrong for poor Meghan, and we can theorize all we want, but we may never know what actually happened.
Based on the map from her tracking app, it would appear to me like something happened in the water and Meghan tried to regain control of the horse in the water. The horse could have been spooked, Meghan could have been thrown partially off or been just hanging on, or have hung a foot up in a stirrup and the horse struggled until Meghan came free.
My husband, my BFF and I were camping a few years ago when some fellow campers came back into the campground, with one rider off the horse soaking wet and bloodied, with the horse soaking wet as well.
We were told the horse was in the creek when it spooked, and the rider came off but her foot did not come out of the stirrup. The horse circled in a panic, all the while dragging the rider and accidentally kicking her.
Accidents on horses happen very quickly, no matter what the rider’s skill level and how broke the horse is.
One hard and fast rule I always follow is to keep my phone and tracking device on my body and not the horse, because if you become separated, you want to have the phone with you.
In this instance though, because the phone was on the horse, it might provide more information as to what happened.
As far as the horse returning back the way it came, that is not a given. Horses are flight animals, and if scared enough, a horse will flee down the easiest path to get away. If this was a place Meghan had ridden frequently before, the horse would most likely be comfortable in that general area anyway.
I wear a helmet every ride, and had one save my life 4 years ago. Even with the helmet on, I still had a concussion and brain bleed, and was Life Flighted to the nearest trauma center. Add water to a situation, and it can very easily become deadly.
I also use one of two apps I have that track my ride and alert a contact should I stop moving. My husband has been as far away as China, and been able to see exactly where I am at all times.
I am so sorry for Meghan, her husband and her family. She passed way too young.