The CHP has primary jurisdiction over all accidents involving commercial vehicles and buses, regardless of whether the accident was on public or private property. It's also quite likely that Alameda Co. Sheriff requested the CHP to take over investigation due to the extensive property damage.
Drew brings up a good point regarding whether the truck brakes were set up when the driver baled out. A fire under the hood could have burned through the lines coming from the air compressor, which would have automatically set up the brakes, however if the trailers were fully loaded and on a downhill grade, dumping the air ( either by the fire or on purpose ) would only work for a few seconds before the brake shoes heat up and glaze over, rendering them useless. Also to be considered is whether the road down the hill was paved or dirt, and if the truck was over - loaded. Any truck that is abondoned by the driver while moving will pretty much chart its' own course. All valid reasons to involve the CHP Commercial Accident Investigation Team.