SAN DIEGO CALIFORNIA REAR-END CAR COLLISION: Is a person who rear-ends someone in a rear-end car accident always at fault under California law?

Generally yes.  However, there are some rare exceptions.  If the person who was rear-ended suddenly and unreasonably applied their brakes (which means for no good reason), and there are witnesses to substantiate this happening, then the insurance company that represents the person who rear-ended the car in front of him or her will generally deny liability and blame the accident on the person who applied their brakes without any good reason.  Remember, if traffic slows ahead of you and you have to suddenly apply your brakes, then that is with good reason. Anything less can be considered reckless driving.  So in a nutshell, a person who rear-ends someone else is generally 99% at fault with some narrow, albeit, rare exceptions.