Horses don't always behave like they should so a horse can, out the blue, decide to act like a twit.
My youngest kiddo arrived early for her lesson so my trainer told her she could go grab her very ancient, deadbroke pony from the pasture. Pony saw her, spun around and kicked her. The pony had NEVER so much as threatened to kick anyone before!
I had a lesson horse that got mad that I was insisting that he needed to pick up speed at the trot and took off at a dead gallop. I will never forget my trainer very calmly saying "Oooook.... we didn't need him quite that fast, lets slow down a bit". I've NEVER had that horse do that again (trainer is now a very good friend and lets me ride him whenever) and I even trust him with both my kiddos.
My kiddos current lesson pony is known as the "one speed wonder" (his one speed is a dead stop), I've ridden him many times and it's the same ride each time... go, go, go, I mean freakin GO!! He makes Western Pleasure horses look speedy..... which is great for beginning riders but I'm falling asleep up there! My horse was on stall rest and the trainer wanted some company on a trail ride with a fresh horse so I took him since the deadhead would make a great, steady, calm companion horse, right? Nope.... took one look at a LEAF, jumped the leaf and took off at a gallop. I nearly fell off laughing at him since the fresh horse was just standing there going "what is your problem dude... it's a LEAF!" but boy was I grateful I was riding and not my kids!