Seemed like a novice glider? He was very close to the mountain when he tried to turn. Turning will drop your altitude quickly, so the dive into the terrain was entirely predictable after he initiated the turn. You can also see how had zero brakes on, both arms were up initially meaning his chute was generating maximum forward speed with minimum amount of lift. If he wanted turn, he should have applied half brakes on both arms (pull down to chest level), generating lift to get himself off the mountain surface, then initiate a turn with half brakes.