The Economist explains
The problem here is the extraordinary level of interdependence within the financial markets of the rich world. According to data from the Bank for International Settlements, American banks had just $47 billion in exposure to Italian institutions (including the sovereign) as of June. That's a digestable amount. France, on the other hand, has $416 billion in exposure to Italy. That's very bad news for France, but it's also very bad news for America; American banks have a total exposure to France of some $271 billion. Trouble in Italy is manageable, from an American perspective. Trouble in France makes Americans nervous. But that's not where it ends. British banks have a total of $305 billion in total exposure to France. And American exposure to Britain is close to $800 billion.
Comment: That's why it is so silly to rate individual countries. What should be done is a rating for the global banking industry and be set at junk.