It wasn't an edifying sight was it? Horse was being difficult but who knows what had upset it in previous rounds. And her reaction was less than optimal in those circumstances. Her shoulders were up around her ears, she wasn't using her legs properly. Easy to say but it's basic horsemanship to sit quietly and calmly and try and keep your head in those situations whatever else is going on around you. Especially at Olympic level! But as others have said, we obviously missed the incident or cause of upset that started it all.
What upsets me though is that those photos of her striking the horse, visibly upset, will be beamed across the world and undermine the hard work of the Olympic competitors in all equestrian disciplines who uphold the highest standards of horse care and welfare.