Here we have a beautifully illustrated example of Newton's First Law of motion involving shopping carts. Did some force push those carts out the back end of the trailer? Not at all.
Newton's First Law tells us that "an object at rest will remain at rest and an object in motion will remain in motion, at a constant velocity unless acted on by a net external force." What's happening in the video is similar to the old tablecloth trick. You can pull a tablecloth out from under a pile of dishes if you do it quickly, because, while a force accelerates the tablecloth, as long as the tablecloth doesn't apply a significant force on the dishes, they'll stay where they are, in accordance with the First Law.
In the video we replace the dishes with shopping carts and the tablecloth with a tractor trailer. The truck then accelerates out from underneath the shopping carts. Now, if the carts had no wheels, the friction between the carts and the floor of the trailer would probably be sufficient to accelerate them along with the truck. No such luck.
Also, if the truck and carts were already moving together at a constant velocity when the back of the truck came open, the carts wouldn't fall out. In that case the shopping carts would simply continue in their current state of motion (just like they do in the video).
Hopefully contemplating the simplicity and elegance of Newton's Laws of motion will be a consolation to whoever got chewed out over this embarrassing mishap!
Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.