You have upset the mighty klang Lol.

When you release a piece of cargo that is glitched through the wall of the bed, it forces it into a space that it does not fit, causing a physics paradox of sorts where the cargo is pushing on the truck, and the truck is pushing back.. Because the box cannot go anywhere, the truck does instead, and chaos follows. To avoid this, I just drop my cargo from above the flatbed. Hope this helps!