Horses have a brain, they are not a machine. Congratulations if you have gotten a horse to be 100% consistent. I believe that one can drastically reduce their odds, but horses spin and bolt for reasons other than predators. Yes, they are a prey animal, but they can be triggered by other things, even if they’ve been desensitized, trained, etc. Europe (broad statement and weird statement) does have boars, wolves, bears, wild cats, snakes, and a few others that horses could be frightened by. A rider at another local stable was recently thrown from her horse. It was a rider with a very well trained horse that had gone down those same trails for years. They came across two wild hogs, and the horse reacted in a way that the rider came off. The forest is very active at the moment with animals, loggers, and people doing various activities.
I agree the trail riders need the best prepared horses, and potentially more so than competitors in other environments. That is part of the risk of trail riding, hiking, mountain biking, and remote outdoor sports. The “medical rescue” isn’t always easy. People do their best by bringing a cell phone, GPS, riding in groups, or notifying someone of when they left and expect to return. No one wants a loose horse, and loose horse could certainly be a disaster.
There are some horses that I really cannot imagine falling off of, or even what they’d spook at because they are so well trained and sound of mind, but I always keep the thought that they are a living being in the back of my mind and that there is that chance. Even if it is .001%
I do see some people that I deem “braver than I” that take a green horse out on the trails with little to no prior training…like barely broke under saddle. I do think that is dangerous. This fall off of this particular horse did prompt me to return to some training elements and desensitization aspects of his training.