How?

Friend did that back in 2000 or so. To unload at General Motors parts warehouse in Langley, they want you to slide your tandems all the way to the rear.
Then they headed over to Richmond, BC touched the brakes for some traffic slow down. Tandems came flying out the back and took out an oncoming Jeep.

 

I once forgot to put the pins back in on an extendable step-deck after a delivery at a coal mine. It was all fine and dandy on the road down, but then there was an uphill to the exit and...

BANG!

Never did that again.

 

Not always the case. Had a trailer that lock pins were air valve operated. Air valve failed and I was lucky enough to be in Louisville, KY going over river in slow traffic and stepped on brake..Tandems slid all the way to rear and gave me a heck of a scare!!!

 

The valve failed and applied air to release the pins?

 

Yes.

 

Doing a local gig in NC in 2000, 10 wheel Daycab, Empty 53' trlr with tandems forward, I somehow released 5th wheel SLIDE LOCK, so accelerating from and slowing/stopping at Lights was a bit jarring until the LIGHT OF REASON (no, not an Officer), lit up the "Common Sense" and I noticed Switch position.
Glad Truck had no in-cab Release 5th Wheel Lock (as a later OTR truck did to equal no story then).

CHEERS!!