Video didn't show enough of horse before he came after man though, particularly the ears/face/eye.
Lots going on, food nearby, and horse was not in chill mood from what could see of him.
More than likely was giving off "quiet vibes" where horse almost mentally closes off, like brick wall coming down over mind. When they get that quiet vibe, usually will do something like this.
Very watchful and still all of a sudden, not alerted to something outside area, but watchful.
Horses make me a better person.