Back of the Sprinter is a vacuum zone

So we have found something quite interesting and something that has happened a few times to us with the Sprinter van. On multiple occasions we have left a few things on the back of our vans and driven off. At times one quite long trips. Our latest trip was driving from a San Francisco neighborhood where we placed an empty microwave box on our moto carrier on the back of the van and proceeded to drive 50+ miles down to San Jose at freeway speeds only to find that the box was still on the back and actually had moved very little.

As a van moves through the air, it pushes air out of the way, creating a higher pressure zone at the front.

As the air flows around the van, especially towards the back, it can't always follow the sharp contours. This causes the airflow to separate from the van's surface. This separation creates a region of low pressure (sometimes referred to as a "vacuum") behind the van. 

This is the magic that causes this to happen. It’s literally sucking things to the back of the van.