[Kalimac]

I'm tempted to quote Chesterton: "Coincidences are a spiritual sort of puns."

Considering what Frodo's just been through, that's a perfectly natural sort of dream to have; it would be stranger if he wasn't having bizarre nightmares. Plus, we have to remember that the Black Riders have already been in the Shire, and he knows it - in his dreaming, he might be remembering Sam's account of the Black Rider's conversation with the Gaffer, so there's a factual basis for the image. But of course it was also a portent that the influence of the Black Riders and their cohorts would damage the Shire.

Unlike the Orthanc dream and the dream in the house of Tom Bombadil, which are a little more purely visionary, I'd be inclined to chalk this one up to fortuitous coincidence. I'm not saying the dream didn't have deeper meaning, just that along with that deeper meaning it also had another source, which was both illness and memory of something that had already happened.