What would happen if the U.S. lost its AAA? Very hard to say. Foreign investors would still have the problem of finding someplace to put their money. I'd be surprised if the U.S. would lose its AAA rating, but say, France and Germany hold on to their ratings. Japan is already AA. It might result in a revision of how foreigners view ratings in general.
Treasuries are actually higher all during the Asian and European sessions, and its only once the U.S. session really gets going that the bond market sells off.
A better explanation is the continued belief that the Fed is defending some level on Treasuries. Admittedly, I thought they would, but the evidence is clear that they aren't. Here are the Fed's Treasury purchases since the program began: