Yes, because they are stocks aka equities and not commodities.. To keep this brief, let’s say a blight/drought/etc.. started wiping out massive amounts of grain crops over the next decade. That would make the price of corn, wheat, etc.. go much higher, right? However, since the crops/yields are shrinking, will farmers be buying more or less John Deere combine harvesters? While the price levels of ag commodities typically move roughly in sync with the ag stocks (i.e- fertilizers, farm equipment, etc..), one needs to understand the underlying supply-demand dynamics of each.

That’s explanation 1. Explanation 2 is simply (similar but different) that they are stocks… and as such, most stocks get sold off in tandem with the stock market as many of the agri stocks are components of various stock indices & when the stock market is falling (because there is more money flowing out of vs, into stocks), the indexes fall & over the past several decades, the major of individual & even institutional ownership of equities (think your 401k) has shifted away from individual stock picking towards indexing.