Maybe ......They invest the money in the wrong places. Such as Mortgage. When the debtors can't pay the mortgage.. these banks..lost the money. Also they borrow money from other Bank, Hedges Fund.

When bank robbers steal all their money.

1) when it breaks: "The word bankruptcy is formed from the ancient Latin bancus (a bench or table), and ruptus (broken). A "bank" originally referred to a bench, which the first bankers had in the public places, in markets, fairs, etc. on which they tolled their money, wrote their bills of exchange, etc. Hence, when a banker failed, he broke his bank, to advertise to the public that the person to whom the bank belonged was no longer in a condition to continue his business. As this practice was very frequent in Italy, it is said the term bankrupt is derived from the Italian banco rotto, broken bank (see e.g. Ponte Vecchio). Others choose rather to deduce the word from the French banque, "table", and route, "vestigium, trace", by metaphor from the sign left in the ground, of a table once fastened to it and now gone. On this principle they trace the origin of bankrupts from the ancient Roman mensarii or argentarii, who had their tabernae or mensae in certain public places; and who, when they fled, or made off with the money that had been entrusted to them, left only the sign or shadow of their former station behind them." Source and further information: http://en.wikipedia.org/wiki/Bankruptcy 2) "Recently, the pace of write-downs by major money center banks has increased. UBS announced today it wrote down $19 billion. Deutsche Bank announced today it wrote down $4 billion. Lehman Brothers was forced to raise over $4 billion in equity capital. Merrill Lynch and Citigroup are expected to write down tens of billions as well. These are massive sums. The question therefore becomes when will investors stop propping up these banks and refuse to buy new equity capital they have been issuing since the housing bubble started these horrific write-downs? One has to believe that these banks cannot continue to go to market to receive equity infusions in an attempt to bolster their balance sheets. At some point, investors will say 'enough is enough.' It is when the investing community at large refuses to take on more losses as these companies write off more and more in losses that we will begin to see major banks fail or be bailed out. To date, there have been four major financial firm failures and bailouts: Bear Stearns in the U.S., IKB and West LB in Germany and Northern Rock in the UK. All of these events were related to the meltdown in global mortgage markets in some way." Source and further information: http://www.creditwritedowns.com/2008/04/bank-bankruptcy-or-bailout.html Further information: http://en.wikipedia.org/wiki/Bailout