Pulled up behind this truck at a stop light and noticed a whole lot of new damage to the tailgate. Sent a shiver up my spine.

maybe someone does not like it,s owner.how do you know it is NEW damage?

Removing the trailer hitch ball was probably the right thing to do.for everybodys sake.
we dont know whats on the other side of that truck either

Looks like a nice clean arc from where the safety chains limited travel of the hitch.

clean metal, no rust

Possibly the coupler let go, the truck braked, the trailer rode up as high as the safety chains would allow and carved the tailgate. The wiring harness has been pulled and neatly tucked back.
Steven Spielberg

Nope your all wrong. The guy didn't have a receiver and didn't have far to go so he figured the safety chains and light plug would be just fine. Which it was till he had to stop....

Trailered with tailgate open is my guess

Yep, tailgate down and it was hitting the crank for the trailer jack.

That's what happens when you use a pinstripe brush with bristles that are too stiff.