SAN DIEGO CALIFORNIA COMMERCIAL TRUCK ACCIDENT: If a commercial truck driver jackknifes on a California freeway, does that mean the commercial truck driver is automatically at fault for any resulting automobile accident?

No, not necessarily. 

The fact that a commercial truck jackknifes on a California freeway is not, by itself, complete proof of truck operator negligence (legal liability), because many accident situations can present difficulties in which there is no practical way to avoid jackknifing. For example, operation of a commercial truck that has jackknifed may be held to be non-negligent where the jackknifing was due to unforeseeable slipperiness of the road, or to an abrupt turn undertaken to avoid a motorist or stalled truck.  Each jackknife commercial truck accident is unique, and you should consult with an experienced California truck accident attorney.