The $134.5 billion in US government bonds would have represented roughly a third of the Japanese’ debt holdings. As such, any attempt to liquidate them openly would have resulted in a panic- and a potential collapse of the Treasury market.
http://seekingalpha.com/instablog/420590-marli/8420-did-the-japanese-try-to-dump-135-billion-in-us-bonds-on-the-black-market