Read an article today - seems as though CA is a central factor in making this happen:
"The trucking issue with California LA ports, ie the Port of Los Angeles (POLA) and the Port of Long Beach (POLB), is that all semi tractors have to be current with new
California emissions standards. As a consequence, that mean trucks cannot be older than 3 years if they are to pick up or deliver containers at those ports. This issue wipes out approximately half of the fleet trucks used to move containers in/out of the port. Operating the port 24/7 will not cure the issue, because all it does is pile up more containers that sit idle as they await a limited number of trucks to pick them up."