This really makes no sense at all. If you care about what credit rating institutes thinks (I don't think you should, but clearly many people do) then the only effect should be to make U.S. treasuries less attractive relative to other assets because they are now viewed as more risky, something that in turn should raise Treasury yields.
I know of course that the reason Treasury yields have fallen is because the stock market has sold off, and that this could overwhelm the increased risk premium. But since the rating was about Treasuries and not the cyclical outlook for the U.S. economy or the riskiness of stocks, there is by contrast really no reason for the stock market to sell off, and therefore definitely no reason for Treasuries to rally.
This is like car customers reacting to news that one particular dealer defrauds and rips off his customers by shunning other dealers and instead buying more cars from the particular dealer who were shown to be fraudulent.
So apart from the gold rally, today's market movements are completely irrational, except perhaps to the extent they were reacting to other news.