You rear end someone, you’re at fault, period. A line has to be drawn somewhere.
There could be ANY situation (including an idiotic decision by the driver ahead of you) that may result into someone having to suddenly brake hard or stop on a highway. It could be an animal, a person, or a sudden obstacle, etc. The rear driver must be prepared and alert all times to at least avoid a collision.
Driving too close and that too in a passing lane caught up to the BMW. If you could brake, BMW could, too. Most people drive way too fast, tail gate, and oblivious to the surrounding traffic.