Well, it's subtle ---

(B) the downs usually because of unfavorable weather killing or ripening crops too quickly.

One problem is what I mentioned in my above post --- if the verbs ("killing" & "ripening") are this closely joined, then the adverb "quickly" applies to both them. The adverb "quickly" is only supposed to apply to the verb "ripening" --- while it's fine for plants to ripen, it's a problem if they ripen too quickly. The adverb "quickly" is not supposed to apply to "killing" --- it's bad for the weather to kill the plants, and whether it does so quickly or slowly doesn't matter. That is the biggest obvious problem with (B).

Even more subtle --- I can't quite explain why, but "the downs usually caused by unfavorable weather . . ." sounds more natural than the "downs usually because of unfavorable weather . . ." Using some form of the verb "cause" is usually more direct than using the words "because of."

In this sense, this question is not really a solid GMAT SC question, because aside from that relatively subtle thing about the adverb "quickly", there's no real glaring mistake with (B). Here, (D) is right because it has absolutely no mistake, but on the real GMAT, the wrong answers will be more obviously wrong.

Does this make sense? Let me know if you have any questions.

Mike