There will be plenty of behind-the-scenes scenarios and speculation, but the WSJ quotes a source as saying that Bank of America CEO Ken Lewis just got worn out from all the investigations (federal and state) and all the litigation - much of it centered on the now-questionable purchase of Merrill Lynch. Frankly, it wouldn't be surprising if that were the basic explanation. From the NYT:

"This was a necessary and overdue change," said Michael Garland, a spokesman for Change to Win, an investment group that has been advocating for Mr. Lewis' ouster for months. "The onus is now on the board to engage with shareholders to name a successor who can quickly restore Bank of America's credibility with regulators and investors." In the last few months, the bank and Mr. Lewis have become embroiled in several investigations of the deal that focused on disclosures over Merrill Lynch's losses and billions of dollars in bonus payments the firm paid out just before the merger.

Here's the FT's take:

Mr Lewis's announcement, at a hastily called board meeting in New York, took the directors by surprise, says Tom Ryan, chief executive of CVS, who will serve on the search committee to find Mr Lewis's successor. For Mr Lewis, the announcement brings him full circle from the time last October when he was celebrated on the TV programme 60 Minutes as a folk hero for agreeing to buy Merrill Lynch to a political punch bag for his role in the complicated deal whereby Mr Lewis agreed to complete the Merrill acquisition only after his job was threatened by federal regulators.