“It appears that the bulk of the bonds that Japanese investors sold in May were US Treasurys and the roughly $30 billion were a record amount. This follows the $15.5 billion sales in April. May was the fifth consecutive month Japanese investors have reduced their U.S. Treasury holdings and over this period sold about 8 trilion yen. The second-quarter selloff appears driven to a larger extent by worries about Fed tapering and its potential to push up global bond yields, they said. This is what is going to make it very difficult for the Fed to slow/stop buying Treasury securities. When they slow/stop, there will be even more upside pressure on rates, with very serious sellers.”
http://www.economicpolicyjournal.com/2013/07/japanese-are-bailing-out-of-us-treasury.html