No, rear-ending someone is not ALWAYS your fault.
Last year on 85S before Saratoga exit, a car cut off the car in front of me, she had to slam on her brakes. I slammed on mine, stopped 2 inches before her car. The car behind me however didn't stop in time, hit me and the force propelled me into the car in front, even though I had already stopped completely and my foot was still on the brake.
I explained this to the CHP and he assigned me no blame. You could tell that I had completely stopped because the indentation that my license plate bolts made on the car in front were lower on her rear bumper, than if I had not been at maximum braking.
I didn't have to pay for anything, everything was paid for by the woman who rear-ended me, causing me to rear-end someone else.
Just think about it, if a garbage truck rear ended you, causing you to rear end the car in front, and you were completely at rest, would you have ANY blame? Not at all.