From Jeffrey Friedman at Critical Review's very interesting "Causes of the Crisis" blog:
Many banks had invested heavily in triple-A rated tranches of subprime mortgage-backed securities, and when delinquencies and defaults on subprime mortgages began to spike, the price of these tranches began to fall, calling into question the solvency of banks that had invested in them….
How can the banks' investments in subprime mortgage-backed securities be explained?…self-interest, i.e., "greed," is always the most popular explanation among economists—and the general public. So a new idea took root: Far from being irrational, bankers knew how risky these investments were, but made them anyway because they were paid big bonuses for short-term profits.
This "executive compensation" theory of the crisis is now the keystone of the conventional wisdom, having been embraced by President Obama, the leaders of France and Germany, and virtually the entire financial press. But if anyone has evidence for the executive-compensation thesis, they have yet to produce it. It's a great theory. It "makes sense"—we all know how greedy bankers are! But is it true?
The evidence that has been produced suggests that it is false.
For one thing, bankers were often compensated in stock as well as with bonuses, and the value of this stock was wiped out because of the investments in question. Richard Fuld of Lehman Brothers lost $1 billion this way; Sanford Weill of Citigroup lost half that amount. A study by Rüdiger Fahlenbrach and René Stulz  showed that banks with CEOs who held a lot of stock in the bank did worse than banks with CEOs who held less stock, suggesting that the bankers were simply ignorant of the risks their institutions were taking….
Perhaps the most powerful evidence against the executive-compensation thesis, however, is that 81 percent of the mortgage-backed tranches purchased by banks were rated AAA, and thus produced lower returns than the double-A and lower-rated tranches of the same mortgage-backed securities that were available. Bankers who were indifferent to risk because they were seeking higher return, hence higher bonuses, should have bought the lower-rated tranches universally, but they did so only 19 percent of the time.
Mike Flynn from Reason Online back last October on the causes of the crisis.