Unlike say track running, fell running has lots of ups and downs and rough ground, so there's more passing taking place. If the runner in front insists on taking the racing line regardless, then theres a chance of the passing runner getting injured, or the slower runner suddenly getting collided with. A bit of courtesy and communication makes sense, and there's no point in holding up a faster runner.
Being a fat lad with flappy feet, people seem quite keen to get out the way on my out of control descents!