Looked like the horse choked down to me but he could well have had other problems that were unseen. I know this can happen from experience, about twenty years ago I was driving a horse in a trial at Cessnock when all of a sudden he was unresponsive in the mouth, then started to wobble about and then crashed over the running rail ( we were still using running rails in 1994) The horse had died on his feet, presumably from a heart attack, he had been doing well in all his work, had qualified the week before quite easily...so yeah stuff can be going on inside the horse that nobody knows a about.