In my experience, the most common reason for crankarms loosening is a burr forming on the crankarm taper. Each time an the arm is installed, a small amount of aluminum is displaced, building up a small burr at the tip of the spindle. Eventually, the burr builds up to the point where the arm cannot be pressed far enough onto the spindle, regardless of the installation torque. Filing down the burr solves the issue in the vast majority of cases. This is far less of a problem with forged crankarms than cast crankarms, which typically use softer, lower strength alloys. It is fairly common on low grade, box store bicycles. In the OP's case, the impact method used to re-install the cranklarm may have been sufficient to overcome any burr, which a progressive torquing may not have achieved.