Comment by Animats

There never was a container shortage worldwide. There were a lot of containers in the wrong places. They were piling up in West Coast US ports and scarce in China. It was more cost-effective to take ships back empty than filling them with empty containers to go back. Some of this was corrected by applying port fees to shippers who didn't take enough containers back.

This reflects the US's lack of exports and China no longer wanting unsorted US recycling.