Slowing Growth in U.S. Freight Shipments?

According to joc(dot)com, the Cass Freight Index shows that United States freight shipments increased only .3% this March 2015.
Reasons cited:

• Economic slowdown
• U.S. West Coast Ports port backups
• ‘Late’ Chinese New Year
• Soft Railroad Car volumes

The article does a good job expounding on each of the drivers of this marked change in the Cass Freight Index.

Are there any additional reasons the Index may have changed? You can read the original article here: http://bit.ly/1b51QcC