TLDW: the likely cause of the crash was low-level aerobatic maneuvering at an altitude of 700 to 1,000 ft, which is too low for such maneuvers. The pilot may have attempted a split S maneuver (see https://en.wikipedia.org/wiki/Split_S - aileron roll followed by a half loop) without getting the nose up high enough, ending up too low. It appears that the pilot had control of the airplane on the video, it wasn't stalled.
TLDW: the likely cause of the crash was low-level aerobatic maneuvering at an altitude of 700 to 1,000 ft, which is too low for such maneuvers. The pilot may have attempted a split S maneuver (see https://en.wikipedia.org/wiki/Split_S - aileron roll followed by a half loop) without getting the nose up high enough, ending up too low. It appears that the pilot had control of the airplane on the video, it wasn't stalled.