Every boater's nightmare.
I’ve been hauling boats in and out of the water for 45 years, every weekend some summers, and honestly never had that nightmare.

Even someone with zero experience should understand Newton always wins, inertia is real. If there’s some reason you can’t hook it on with the winch, then at least lash it to the trailer with a bit of line!
To have punched the accelerator coming out of the water like that, and to not see the boat sliding off the trailer in the rear-view… these guys are a special kind of stupid that you don’t often encounter in daily life.
I have had a boat come off a trailer once, after sailing on an incredibly windy day, gusts in the 40 - 60 mph range. I had pulled the boat from the water and parked it in the lot to pull down the rigging and pack it up for travel, when a gust of wind hit us so hard it lifted the boat off the parked trailer and threw it onto its side in the parking lot.
Thankfully, a few crazy hikers or cyclists were still out in that weather, and were able to help me lift the boat and muscle it back onto the trailer and tie it down. Luckily it was just the roto-molded I use for training kids to sail, and not glass or my good mahogany woodie boat, so no major damage was done.