It's a Bank Balance Sheet Race

from Calculated Risk blog:
Imagine a bank holds a RMBS (Residential Mortgage Back Security). Forget about tranches - just imagine the security is based on 100 mortgage loans. All of the loans are current, but the security is actively traded, and the price falls to 50 cents on the dollar because investors believe that there are many default (and losses) coming. The bank has lost 50% immediately. The bank holds the security, not the loans - so it is the change in the value of the security that hits their income statement.

Perhaps the bank believes the most profitable thing to do is just keep the loans in its own portfolio (fair value accounting principle called "highest and best use"). Now the bank also has a portfolio of 100 loans with exactly the same characteristics as the RMBS. The Fair Value estimate for income producing loans for which there is no available market or counterparty will be based on the Income approach (discounted future cash flows). As before all of the loans are current, so the bank takes writedowns based on estimates of discounted future cash flows (they are being held to maturity). As the losses, both current and future, become estimable, the bank takes the writedown.
The Fed estimated both future losses and future earnings. As Dr. Roubini noted, there will be a race between losses and earnings - and if the Fed overestimated earnings or underestimated losses, the banks will need additional capital.

Strange thing to bet our future on!