It's because the shipping containers belong to the shipping company, and are rented out for the trip. So the shipping company will have a bunch of empty contai

It's because the shipping containers belong to the shipping company, and are rented out for the trip. So the shipping company will have a bunch of empty containers at the port of landing (destination), and needs to send them back to the port of departure to be filled again. If trade between the two ports is balanced, then the containers will always be full regardless of the port of departure. But since trade is not balanced, they wind up having to send empty containers on the "return" voyage. Which is pure loss for the shipping company, so they are willing to heavily discount routes that have empty containers transiting. Which are these routes does change of course, but they are often those originating in the EU / US and arriving in Asia.