If you watch the video carefully, this horse has a problem in both stifles and something extra going wrong in her right hind. In general, this horse moves short behind, doesn't even come close to tracking up, and sinks a little in the hip each time he takes a step. It's signifigant that he does not travel straight, but prefers to carry his head and shoulders to the right and on a separate track of his hips and hindquarters.
In addition, he has a pronounced flinch each time he brings his right hind forward and steps off.
I suspect ongoing stifle and/or hip problems on both sides, more pronounced on the right.
I am not a vet, and I don't play one on TV, but that is what I see as a moderately experienced observer.
I am taking into consideration that the horse is green, unshod behind and working on hard, uneven ground. However, the things I observe stay consistent despite gait and direction and cannot be completely dismissed.