Training horses starts with understanding how their minds work. You have to understand what is logical to the horse. The horse's mind does not work the same way as yours. They do not associate events or a sequence of actions in the same way we reason that things are related. To train a horse, therefore, you have to understand how horse logic works and base your training on that.
Horses are prey animals.
They are in an undesirable position in the food chain and they know this. Their eyes are on the outside of their heads so they can see danger coming from any direction. When we approach a horse, it has no way of knowing what our actual intent is. It can only observe our actions and make a decision that it is safe to stay put or safer to flee. When a large cat approaches a group of gazelles as a hunter, the whole herd will start running and try to escape until one of them is killed.