Fire safety is a huge aspect of any construction in the U.S. It is unbelievably incompetent to allow the mass storage of flammable materials beneath any structure without the structure and its fire suppression systems being designed to handle the combustion of these stored materials. There are codes and laws "written in blood" as they say regulating these issues. In any kind of facility, the designers must assume that a fire will occur at some point, because they do. Highway overpasses are designed to resist fire, but not a fire that lasts for hours and is fueled by thousands of pallets.

This fire and its consequences are the responsibility of both Caltrans and the leasholder. But remember, this land belongs to Caltrans, and they allowed these materials to collect under their freeway against all good sense and likely a few codes as well. They could have done something about this, but they didn't. The source of the ignition is pretty much immaterial, because outdoor piles of dry wood are eventually going to catch on fire.