Just Guessing
Maybe he stopped on the ramp (facing the water, I guess) in neutral, engine running, parking brake on, got out to unstrap the boat from the trailer, and while he was out of the cab, the brake somehow realeased?
Or shut off the engine, left a manual transmission not-so-securely in gear as a brake, but it popped out (this has happened to me, sloppily shifting into gear after I shut off the engine: didn't quite engage the gears).
I know, both are stretches, but I'm bumfuzzled. Facing the water. Go figure.
This had to be first attempt at launching a boat. At least, I hope so.