Obviously what has happened is that Lieberman has (quite accurately) perceived that he could go nowhere nationally within the Democratic Party without moving into the lockstep official party position on abortion, judicial nominations, and so forth.
We've seen it all before, of course. But more frequently, Southern Democrats are involved. Al Gore was once pro-life, and John Edwards, during his 1998 Senate campaign, said he'd support a partial-birth abortion ban; in fact, he voted against a ban in 1999, and was unable to fit a vote on the recently-passed ban into his schedule -- he and John Kerry were the two absentees.
Slowly, slowly, the two parties are becoming more ideologically focused. Conservative Democrats and liberal Republicans, both commonplace two or three decades ago, are becoming endangered species.
Abortion can do that to a politician.
Not surprisingly, the issue that has caused this divide is abortion. Ironically this has happened because the "Pro-choice" crowd are fanatics who refused to live with the limits proposed in Roe V. Wade. If they had accepted the trimester scheme, the pro-life movement might not have held its ground.