From my experience here’s what I could tell from the video. The hill wasn’t tall enough. The slope of the hill was shallower than the glide ratio of his glider.
You see this type of mishap at every training hill. A guy I was taking lessons with broke his arm about the 4th time he made that same type of “landing”. I weighed about 50 pounds less than him and had a double surface glider. I made it to the bottom of the hill everytime and he crashed everytime. Even with “training wheels” he still managed to break his arm.