You cant blame it on the groundie, how do we know he had the liberty of letting the piece ride, who know what was at the base of the tree? He tied the piece off a few to many feet above his notch, this added to the length of the drop before impact on the line, in essence greatly increasing the shock load. He should have tied a half hitch a few inches above the notch then finished it of with a Running bowline a few feet above the half hitch. By just reducing the drop by a foot he would have decreased the shock load by at least 50%.....