He was kicked out of the Democratic party back in August of 2006, which must have been a very personal and humiliating experience for him. I'm not sure whether to blame the activists for alienating him so profoundly or give them credit for seeing into his character earlier than others.
The key date in Lieberman's alienation from the Democratic Party was in 2004, when he went from 1st in the polls to 5th place in N.H. in the Democratic primary, and wasn't handed the nomination for the Presidency he felt he deserved. At that point, he transitioned into full spite mode and started attacking the Democratic party and its elected officials in every interview and media appearance he was given. Not surprisingly, he started getting a lot more on Fox News.
The 2006 primary was the activists' gamble they could finish him off. It failed. At that point, Lieberman was totally liberated to become the power player he couldn't be as a Democrat. He was bound by campaign promises to support the Democratic caucus, but that only lasted a few months. However, his future prospects look worse than he ever could have imagined.