Actually, when this 'made the rounds' the first(?)time recently, someone who'd actually been there commented that the horse was NOT green; had been shown quite a bit. Not to say this couldn't be the 'cause' of such an occurance, just that it apparently wasn't the case this time.
I suspect that it isn't JUST the mini breed ring where horses who really AREN'T trained to a proper level for competition are 'in there', anyway...and could and DO present a danger to others. When LOTS of the horses in the ring are like that, it's a disaster waiting to happen.
It seemed to me that there were mistakes made all around. I agree that the horses should not have first been asked to walk, but to come into ring center ASAP, while glancing over their shoulder first....then staying close to the judges' stand, while ring personnel work to keep the runaway on the rail. Many horses will 'run themselves out' fairly quickly, and be ready to slow down/stop on their own, if they can just be kept clear of others.
Advice from longtime drivers is almost ALWAYS to stay in the cart, not to 'bail' or dismount the cart. I had first hand proof of the reason for this when I was run over from behind, while on a dirt track in a SECTION(one square mile) of open ground, by a friend's runaway horse(who until then had been 'trained' ONLY for the breed ring, and was actually typical of that type of 'training'-though he'd won big in the breed ring, including at World....).He, out of the blue, 'blew up', bucking, and cutting sharply back, dumping her from the EE cart. I'd been ahead of her--stopped, got out of my cart to see how she was. As she approached me, at my horse's head, we were momentarily distracted; her horse, now just galloping(he never appeared to be in abject terror--just running) was circling out in the pasture, ran RIGHT OVER ME from behind, knocking me down and stepping on the back of the calf of my right leg(which had recently had knee surgery!), and rolling the cart over me--and of course, causing me to lose hold of my horse. Result? TWO horses loose with carts attached in the pasture! (All ended well, thank God...my mare just headed back for home; her horse ran far enough to tire, and was stopped by passing good samaritans before he got back out near traffic....but I surely learned MY lesson!)
Margo