Was Ken Lewis just a bit burnt out?

Why is Ken Lewis going? And why is Ken Lewis going now, when there's no clear person to take over from him?

Obviously, it may have something to do with Andrew Cuomo and the SEC. John Carney points out that Charlie Gasparino reported recently that 'friends of Lewis' said he wouldn't step down if he were sued personally by Cuomo over the Merrill acquisition, only if he were sued by the SEC. That is now a possibility.

Equally, however, Lewis's departure may simply be because he was a bit frazzled. In his parting memo he said he usually feels energized after ending the summer in the mountains and reflecting on the 'bank's challenges and our strategies to meet them.'

This year, Lewis said he came down from the mountains thinking his job was done. But the Wall Street Journal says he came down from the mountains and told a 'person close to him' that he was "really tired of all the mud that was being piled on him."

In a sign that he was cutting loose (and that he has remarkably fast hair growth), the Journal says Lewis then grew a full beard on Labor Day and came into work with it.

Last night, his wife said he wasn't talking to anyone because he was 'exhausted.'

Lewis's exit raises big questions about his replacement. The Charlotte Business Journal has a look at his successors, which it says amount to Brian Moynihan, Dan Montag, Joe Price, Barbara Desoer, Sallie Krawcheck, or an outsider - Al de Molina, the CEO at GMAC Financial Corp.

Needless to say, Montag looks like the best bet for the investment bank.