Okay, just a short ways into the video: Why are people trying to grab those reins or get in front of the horse when it is moving that fast?
My guess is to try and stop it, it looked like many of them weren’t particularly experienced, but hard to tell from small video and no sound. You can often stop a horse by getting in its way and waving your arms, but you can also get run over as happened to several people there. It looked like at one point a guy was able to grab the harness and stop the one horse and then another got spooked.
I’ve had horses almost come over or through a 5 foot gate when I was working Western Gymkhanas. That was with a rider on it. You did not open that gate though until the rider had the horse stopped and under controlor you might have another 20 or 30 or more horses going wild in the holding pen. Pretty much even if it meant taking a 1000lb horse in the face. Same reason most classes didn’t allow more than one or two horses in the ring.