Truth be told, in some states, (like VA) if you're the driver that rear-ends another car, its your fault, period. What caused them to stop is irrelevant in the view of these laws, you should always have enough following distance and enough control of your car to stop before you hit someone. In this case, they'd say you were going too fast for the conditions, (blind curve) and didn't have control of your car.
I'm not saying you're a bad driver, and I'm not saying the U-turn person doesn't share some blame, but legally, she may not! She also probably had no legal requirement to stay on the scene since she wasn't involved in the accident itself.