This piece postulates an explanation, and it's intriguing. I like even that someone tried to answer the question, which is one I've wondered about.
Why Is Paul Krugman So Hostile To Barack Obama? was posted on February 11th and written by Cass Sunstein. What is not noted (and that I did here) is that Cass Sunstein is an advisor as well as a colleague who also teaches law at the University of Chicago Law School.