[聯邦儲備銀行減息]希望有心人可以為我解答 謝謝

It is because the investment banks that invested large amount of money in the assets backed securities on the sub-prime mortgage will have outstanding commercial papers to roll over ( borrow money). However, market lacks in confidence to lend them money as their default risk have risen.
Feds can lower the interest rates so that market liquidity can be increased and Feds can act as lender of last resort to lend them money, in order to restore the market confidence and stabilize the financial system.