It is said that the Nazgul are neither living nor dead so i think it has nothing to do with the connection to ghosts that was mentioned earlier. I agree that they retreated and were not driven off. I think they were trying to wait for a time when they could attack with all nine Nazgul. They knew that they could'nt get the ring at that particular time so they left, besides they had stabbed Frodo with a Morgul-Blade and why fight now when you can come back and fight later when the ring-bearer is a lot weaker.
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"One more soldier dies, and when he gets to Heaven, to St.Peter he will tell, one more soldier reporting for duty, I've served my time in Hell."
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