Now then, I have eliminated two of the three possible choices the horse has in this specific situation - leaving my area AND challenging me for leadership. That ONLY leaves complying. In nature, in a herd situation, there is no other option - he simply must comply in keeping with the herd dynamic. AND I was enforcing the herd dynamic, fear rose. Namely the fear he was not going to be able to find a herd to be a part of - thus he would be more vulnerable to predation and would be denied the socialization inherent in being a herd member.
Horses look to the herd leader for justification for an action.
When something spooks them they follow the lead of the lead horse. If they flee and look back and she is not fleeing, they circle and rejoin the herd. If the lead mare is fleeing, they are ALL fleeing and they will go for as long and as far as she does. Security or the removal of fear is rooted in the herd dynamic principle. As long as there is a clear leader, there will be clear compliance.