I just fixed several things on our garage door, and this was one of the problems. There is an electrical trip mechanism. If this isn't tripped, the garage door will reopen automatically. Climb a ladder and look for 2 turnkey dials on the side of the opener. One adjusts the stoppage of the open position; one adjusts the stoppage of the closed position. You'll need to mess with the dial that controls the close position. The trigger to keep the door closed isn't being tripped. Try turning the dial one way, if that doesn't work, turn it back the other. You'll probably spend a good 30 mins or so messing with it (open, close, open, close, open, close...it's a pain) until you find the right stop point.