The containers, when loaded, might be anywhere from a couple thousand up to twelve thousand pounds each. We have crews of people loading them on flat level ground so it would be difficult (to say the least) for a single person or two to push them out the door while the plane is in flight.
Assuming everything happens at reasonable altitude, pilot just starts descending the aircraft and the containers become lighter, all the way to weightlessness (as in "Vomit Comet"). All is left is just to push them out.
