Based on the increasing deficit and decreasing confidence that the Democrats & Republicans will actually do anything about it.

"Because the US has, relative to its 'AAA' [top-rated] peers, what we consider to be very large budget deficits and rising government indebtedness and the path to addressing these is not clear to us, we have revised our outlook on the long-term rating to negative from stable," the agency said in a statement.
http://www.bbc.co.uk/news/business-13118834

Chip H.