Similar story from many years ago ( no pics ) at a steep concrete boat ramp on Lake travis, a medium sized PU backed in to launch. Conditions were just right ( or wrong ? ) that the alge was severe. He had it in park and set the parking brake.
To his amazement it started moving back into the water. Once it got moving it kept going just like in your pic above...
I'm a believer...when I mounted a utility frame on my front bumper I drilled a hole for a trailer ball...Launch forward keeps the drive wheels ( rear drive ) on dry land.