You're right, the runner is not down if nothing but his feet or hands touch the ground, so if he falls on someone and bounces up, the play is still live (unless IW
). Anyhow, the way to think about landing on a pile is the same as if the runner was being stood up by a/many tackler(s). The play is dead when his forward progress has stopped. You blow the whistle to prevent the runner from getting injured with further play.