There is a large backlog of freight exports from China, as shippers re-direct freight from ocean shipping to air. One of the major factors is that some Chinese airlines are switching more than 20 freighter aircraft to transport relief medical material to West Africa for the Ebola crisis.
In addition, sea ports are congested, which also helps to force sea freight to air.
While the west coast port situation, combined with the holiday season, is undoubtedly adding to the problems, there are also structural issues in the USA which may prevent the congestion from easing significantly in the short term.
Larger ships with bigger cargo volumes, gate hours, labour staffing and productivity are all problems.