The U.S. wants to prevent Japan selling more U.S. bonds to stabilize the yen, which would send yields higher and interest rates up for U.S. consumers.

July 31 : The U.S. Treasury intervened to support the yen on Friday through outright purchases, the Financial Times reported, joining Japan in its efforts to boost the currency…

It was Washington's first yen-buying intervention with Tokyo in more than a decade as it languishes near 40-year lows.