I also think you let yourself overreact to the words "Fell winter",
Lmp 
We are talking about awful winter in local parts of Rohan, or at most in the whole Rohan and, say, bits of Gondor. Really awful winter, but that happens and used to happen in our history just by very unfortunate combination of weather effects. Besides I daresay volcanic eruptions would be much more uncanonical than anything else we might conjure.
What
Nog and especially
G55 said was exactly what I was imagining (although Rohan I daresay can't be compared to Finland, but evil winters can come also elsewhere. I'd compare Rohan perhaps to Ukraine, lot of it actually is similar in terms of climate, vegetation, and surrounding biotopes, with perhaps the difference that Rohan is even more land-locked, which makes the climate less mild. Imagine, you get mostly eastern winds from the desert-ish lands beyond Anduin, and they get stopped by Misty Mountains, and if you get a dry summer and, by unfortunate accident, a very early dry frost, you're in for a bad start. Anyway, point being, a bad winter in such area is imaginable, and let us bear in mind we don't probably have amazingly extensive farming).
I'd actually be for
G55s account being used as the real explanation, as that sums up a nice chain of events that might have caused the main problem: bad harvest, early frosts, some snowstorms for isolation, heŗe we go.