So, How Did The Dagger Appear In Herman Cain's Back?

It's like an Agatha Christie mystery: just way too many suspects. Cain is the only candidate who looks and sounds like a Main Street Republican - a successful businessman - and therefore he is an intolerable threat to way too many people:

In the case of Cain, there was probably an incentive for any of his rivals to have leaked the sexual harassment story. But as Kennedy notes, that’s also true for just about anyone in the Republican establishment, even those who aren’t aligned with a particular campaign, who have plenty of reason to believe that Cain would be a disaster for their party as its nominee. So there’s no shortage of suspects. And, of course, it’s always possible the story came from somewhere else entirely.