What drove an increase in the value of the yen between 2008 and 2011?

Previous to 2008, the US Treasury Bills were bought since they were offering yields of 3 to 4% because of changes in interest rates. Once the financial crisis hit, these bills no longer held the similar value, resulting in financial institutes vending off their Treasury Bills. Though, the debt which was engaged out during the period of "carry trade" still occurred and the financial institutions had to reimburse those arrears back. Consequently, they accepted Yen to remunerate their debts and augmented the demand for Yen. The growth in demand for Yen led to a proliferation in the price of the Yen.