Seems to me like he thought the pallet jack was meant to work like a spatula to scoop things up off the ground, and tried to force the box against something so it would stop sliding around and get on the jack.

My best guess he thought it was empty, didn’t want to pick it up, so tried to cram it under the shelf with the pallet-whatever-it’s-called. Then felt betrayed that there was actually something in there like the idiot he is?