Short blog post means big generalizations, but...
Post-impeachment, post-Bush selection, post-9/11 was a weird time in American politics (I suppose a specific weird time, it's always a weird time). One thing people forget about the impeachment era was that it was basically The Left (sometimes actually The Left like The Nation magazine writers and sometimes people who found themselves being branded The Left because of this) who defended Bill Clinton in the whole Monica Madness era (and before). Mainstream media (hi New York Times!), columnists, cable news personalities, all the respectable prominent "centrist" Democrats, were falling all over themselves to condemn that nasty Bill Clinton and his nasty penis, and Ken Starr was treated as the second coming of Jesus in respectable DC circles. It was a weird time in which the crazy left were actually the biggest defenders of the Democratic party, much bigger defenders of it than the Democratic party itself. It was a time when you wouldn't have been surprised if you woke up one morning and half the party hadn't decided to switch teams and become Republicans. "I was a Democrat before Bill Clinton did nasty things with that woman, but now I don't think rich people should pay taxes anymore..."
And then the selection, and then Iraq, and then Bush's re-election, and the whole Social Security privatization nonsense... It was always the "crazy left" that was trying to make the Democratic party just, you know, be Democrats, and everybody else basically being like "Why can't a Democrat be more like a Republican." Being against the war or against Social Security privatization (the Dems finally woke up on that one, but it took a lot of yelling) wasn't exactly calling for full communism, and plenty of people who thought they were just standard squishy Democrats suddenly found themselves being lumped together with radicals.
So I found myself on the crazy left. I'm genuinely more "lefty" than I was 15 years ago, but even now I'm not exactly calling for full communism. I generally think that usually the best use of my efforts are to pull the party leftward (not that I think I have the superpowers required to do this), not just because I'm more lefty, but because the forces pulling them to the right continue to be powerful and well-funded. Also, if the "crazy" position is a minimum wage of $25 an hour, then $15 an hour doesn't look so crazy anymore (for example). If the best we can ever do is a compromise, then it's best not to start the negotiations with the compromise position.
People get mad about criticizing Democrats these days in a way they never did before. People like Obama and associate "the crazy left" with Bernie, blaming him (and therefore the crazy left) for Clinton's election problems. Maybe I'm wrong, but whatever horrors the Trump administration is going to unleash, the important thing is for the Democrats to draw distinctions, and not just hope for team R to step on enough rakes. "Not as evil as the other guys" just doesn't win elections, even when the other guys are really fucking evil.