The downgrade didn't cause this. At least, you need a rather complicated model to explain how it did. Interest rates fell on US Treasury bonds; this means investors think they're the safest instrument to invest in. Today we saw equities crumble, not bonds.

I would guess it's mostly the evolving European situation that's throwing us headlong into the abyss, with a hefty serving of realizing the USA is run by clowns who are utterly incapable of addressing any important problems. Note that this is beyond simple "politicians are teh stupid," which applies in any country, but in the USA particularly. Europe's facing serious problems, but that's because they're burdened with very tough choices as a result of badly thought out institutions from the 1990s. The USA faces very easy choices, but we're facing serious problems because... well, we want to? Because we have a dysfunctional government? Who knows. Regardless, it's an unforced error.