[anamax]

Fine - why are they on the brink of bankrupcy? A company's stock price does not affect the value of its assets. If folks are cashing in stock and depositing money, that's good for a bank. (However, taking out money and putting it elsewhere is bad.) If their competitors are unable to make loans, that's also good.

[mixmax]

Banks make money lending money to you and me. This is not necessarily money they have themselves, but borrow on the interbank market. How much they can borrow depends on what kind of collateral they can put up, and whether other banks trust them to pay it back. If their stock price goes down significantly their collateral shrinks (they are "officially" worth less and thus have less collateral) and it becomes harder for them to borrow from other banks. This starts a vicious circle where other banks start questioning their ability to pay back. When the bank runs out of money (liquidity) it goes bankrupt.

Simple.. :-)