Lieberman opposes things he supported about three months ago. There is no rational basis to his opposition. Why people think he is acting out of principle is beyond me. He's being a dick because he can. That's all there is to it.
With his disgraceful actions yesterday, Liberman has shown himself unworthy of being an American senator. The senate needs to vote to unseat him, which will give Connecticut a chance to have a special election and replace him with a decent human being.