One theory of mine is that the Nazgûl were rather powerless when it came to actual battle prowess. It is often stated that their main weapon was fear.
Therefore, they are threatened by people whp are fearless, such as Éowyn and Merry, and Aragorn in that occassion, too. Eowyn and Merry no longer feared for their lives on the Pelennor; and Aragorn knew he had to do all in his power to save the Ringbearer. The Witch-King fleeing from Earnur would be another example (and having a reincarnated angelic Elf-lord pursuing him didn't help either).
This would explain at least the situation at Weathertop rather nicely. They were afraid of fire and water, for whatever reason -- the pure physical side is less important here, I think; rather the spiritual connotations (such as the power of the Lord of Water) -- but their being afraid on Weathertop was more due to Aragorn. And it probably was a tactical retreat anyway.
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