First of all, he's in a contested primary. That's the nature of American politics - he was not awarded that Senate seat for life.
Second, and more importantly, in 2004 the Democrats went against all their principles and nominated John Kerry, whose primary qualification seemed to be that Democrats considered him to be a war hero. Why is it surprising that the same political party is willing to throw Senator Lieberman overboard?
"..Democrats went against all their principles.."
Oh, right, all their principles, which are:
1.......... hmm, well, Democrats will convene a summit to decide what their principles are. First though, they must decide where to hold that summit, San Fransissysko, Moscow, Beijing, Afghanistan, Turkmenistan or Bah Stan