You had to crank the landing gear up anyways and you couldn’t take the 3 seconds to look and see you had high hitched it?
Because you've made a typical newbee rookie mistake by not getting out of your truck and checking the clearance between the 5th wheel and the trailer every experienced driver knows before backing and hooking up you back part of the way under then lift up your landing gear about an inch off the ground then back under lock in your king pin then do the tug test this is a never fail system and you'll never ever lose a trailer again. Just be careful not all truck clearances are the same especially if its one of those shunt trucks they lift the trailers up alot higher bottom line is always check your clearance get out of the truck and get some exercise and take a good look before hooking up.
The other thing i forgot to mention the other day is another reason for getting out and checking is some times the trailer can actually be to low and you could cause alot of damage to your truck or the trailer if you don't get out and check it first, 1st rule of thumb always aire on the side of caution.
I think you overrode the fifth wheel. And it found a way to want out off your tractor and so it did. It is totally preventable and yes chargeable to you. It is one of the most basic .. er paranoia of a trucker to be absolutely certain that you are HOOKED to that trailer. ANY thing less you are not hooked. If you want to be spoonfed a explanation how that trailer came off, my thinking is that the fifth wheel is round. And so a overridden trailer kingpin will find one side or the other to slide around and off she goes. I hate to be difficult but this is one thing you will need to learn really well so you don't do this stuff again. Don't you think it was a TIGHT SQUEEZE between that cab and trailer putting on glad hands?
At a Campbells soup plant i made the mistake thinking I was hooked . It was 42000 lbs sitting on my drives , I was able to get it back up .Now I always check visully aswell.
Once you jump your 5th wheel, and spend about 20 minutes cranking the trailer up, you'll get it lined up, and check the height. Once you almost drop one, you'll always visually verify it, and tug it. I've done both.