From the Journal of Commerce:

Container volumes moving through West Coast ports in May were 4 percent higher than during the same month last year, lending support to the theory that higher-than usual cargo volumes are contributing to port congestion.

Jim McKenna, PMA president, said the surge in imports is having a ripple effect in the supply chain. Import containers cannot be processed quickly enough at distribution facilities, so the containers, and the chassis on which they are mounted, are backing up at the warehouses. This is causing equipment shortages at the seaports.

More at the JOC