How?

DCPCFML:
I’d say he wasn’t watching where he was going, turned too quickly to the left and put the nearside into the railings which are just at the right height to impale themselves under the box body. His continuing forward momentum has then caused the railings to rip the box off the chassis, as they’re concreted into the ground and will be the harder of the two to budge. Box body loses and the end result is what you see.

Alright CALM DOWN CALM DOWN