Automatically? No, it's not automatically the fault of the person who rear-ended the other. However, I'd say about 90 percent of rear-end car wreck cases we handle were the fault of the rear driver.
Unfortunately, since it's not automatically the rear-ender's fault, you'll need to prove that the person who hit you caused the accident before you can make a claim against them.
Furthermore, sometimes the front car is truly to blame for an accident. I've written a helpful article that dives deeper into these issues with examples - you can find that here: