People would make money on the downgrade if the interest rates of US Treasury bonds would have risen because of an increased risk foreseen by ratings agencies. One of the major risks in bonds (corporate, and municiple) is interest risk, because if the interest risk rises, the value of the bond itself plummets because now people who had a bond paying 6% could buy a bond paying 10%. So if you're trading bonds on spread, which means you are trading/selling bonds prior to maturation, you could see a significant loss in asset valuation because now people won't buy a bond for it's facevalue if it's interest rate is below the going rate. Banks do this a lot to mitigate inflation on capital. If the interest rates would have increased in US bonds they would have basically just lost a lot of potential liquid to invest, and essentially loan out to it's consumers.
The good news is that the US Treasury bonds did not increase in interest. The SEC is investigating trading by people who have ties to the S&P, to see if they had sold their bonds prior to the downgrade to try and mitigate their losses if the rate had gone up.