What you are going to watch is a horse freaking out over a large boom, after trainer attempts to give a treat to the horse while it is lying down. The trainer has never hurt the horse and he is totally fine even though he bucks a lot in the video. The only reason she yanks on his rope at the end is to simply calm him down, not to hurt, or scare him more than he is.
He is a rather young horse and was being very difficult to handle and that is why she is trying to get his attention back on her. The lady did not yank because the horse spooked, she yanked because he bucked. The horse looks like it was about to calm down before the dog ran in, but the dog ends up running right into his blind spot which makes him buck. If I were holding a bucking horse I would yank, too.
Scared or not they need to get their attention back on me. After she yanks he focuses a little more and calms right down. Spooks can come at the least expected moments, and we are only human. We make mistakes and sometimes yank a little harder than we would like but we don’t do it so that we can hurt our beloved horses. Watch the video till the end!