Trailer is a code tank with current UC tests. My guess is operator error, if it was equipment failure it would have been the jaw on the coupler plate and a visual inspection would have most likely caught it.
How many drivers have you seen at the rest stop, truck stop, etc that fire it up, turn on the blinkers in the cab, look in the mirror, then drive off. Pre-Trips save time, money and breakdowns. Oh and it's the LAW too!
Usually what happens is when the bogie hits the stops the box lifts a little and the bogie pitches forward and down so the front pins come out of their holes and the slider rail comes out of the front brackets. Then the box crashes back down and the rails come down on top of the front brackets, usually closing them up. Makes it VERY difficult to slide the axles back up.