It sounds like you need to adjust your down limit switch and/or perhaps your motor force settings. If the limit switch is not in the sweet spot along the rail it can be triggering too soon or not in time. In your case it could be too far down the rail and thus not stopping/slowing the motor in time. Your door is then being pushed against the floor too hard. Your safety/return force is goes way over limit and tells the opener to go back. While closing your door look at your rail and if its bending when the door hits the ground, this is definitely happening. After finding the sweet spot for your down limit switch, you can readjust your return/safety force per your opener manual.