This happens in Florida
quite often. There is green slime on the ramp
and people with rear wheel
drive just keep on slipping down. The emergancy brake just locks the rear wheels and the car/truck/suv just keeps on backing up. Seen it happen many times and saw many people pulled up.
The solution is to have someone in the drivers seat with foot on brake when loading/unloading the boat
and spin the wheels to cut through the slime.