It is almost impossible for witnesses to know why a car has braked sharply unless they happen to be looking exactly in the correct manner before the car has braked.
Irrespective of how hard I brake, the person following should not be so close as to be unable to stop before hitting me. Therefore their driving is not in accordance with the Road Traffic Act or the Highway Code and they will be usually treated as at fault unless they can clearly show that they were driving within a safe manner and could not be reasonably expected to stop.