The LA Times covered this is an article about his tweets, but in a nutshell: the problem is the shipping companies. They wouldn't pick up empty containers coming back to the ports, so the containers were stuck in the container storage yards outside the ports. Because these storage yards were full, trucking companies couldn't pick up new (full) containers with rigs on location; they had to bring in new rigs. But their aren't enough drivers right now due to COVID and trucking being a generally crappy low-paying job.

The first incentive implemented was to start fining the shipping companies for sitting on their (full) containers. However, this is an indirect incentive, intended to motivate the shipping companies to solve the container lifecycle problem in a market-efficient manner. If this incentive doesn't work, the next incentive will more directly address the problem: shipping companies will be fined for empty containers left at the container storage facilities.