Gail Collins semi-endorses, and adds detail to, my Lieberman's-not-very-bright theory:

Other than that, my favorite explanation comes from Jonathan Chait of The New Republic, who theorized that Lieberman was able to go from Guy Who Wants to Expand Medicare to Guy Who Would Rather Kill Health Care Than Expand Medicare because he “isn’t actually all that smart.”

It’s certainly easier to leap from one position to its total opposite if you never understood your original stance in the first place, and I am thinking Chait’s theory could get some traction. “When I sat next to him in the State Senate, he always surprised me by how little he’d learned about the bill at the time of the vote,” said Bill Curry, a former Connecticut comptroller and Democratic gubernatorial nominee.

I should add that intelligence has a lot of dimensions, and clearly Lieberman is not a moron. But in terms of the ability to understand public policy issues in a specific and detailed manner, Lieberman seems to rank fairly low. He thinks in abstractions, and that's why it's easy for him to decide that a bill with a certain provision is desirable one day and unacceptable the next. That's what's so irritating about Lieberman's behavior. He skipped meetings to hammer out this compromise, he has shown no inclination to master or even grasp the intricacies of the issue, and he nonetheless decided to insert himself at the center of the debate. It's not the behavior of a man who takes seriously the moral imperative of health care reform.