If the driver of car A causes a crash with car B, which causes car B to suddenly stop, and then car B is rear-ended by car C, is A fully responsible for the damages to B, or does C share in some of the responsibility?
The jury decides on a case by case basis from their analysis of the facts. It isn't possible to say in the abstract. A is assumed in the question to have been negligent. C didn't maintain a proper following distance which would be a basis for liability. the jury could also assign some liability to B perhaps concluding that the sudden stop wouldn't have been necessary if B hadn't been negligent.
The proper following distance is based on how far back you need to be to avoid colliding with the vehicle in front of you if the vehicle in front brakes suddenly. If the vehicle in front of you stops as a result of colliding with something, however, it can potentially be brought to a stop more-or-less instantly (in contrast to the gradual deceleration resulting from hard braking), which can easily leave your vehicle unable to stop in time to avoid colliding with it even if you maintained a proper following distance.– VikkiMay 26, 2022 at 6:07