Yes, that was my suspicion after reading your post. I cannot figure out why certain comedians start out kind of OK, then morph into these angry, fulminating caricatures of leftism. Roseanne Barr's former husband called her out for this on Twitter. He asked her if she even remembered when she was a happy, stand-up comedian back in the early 80's. 
Well, being married to Tom Arnold will do that to you.

But seriously... I think middle/old age and fatigue may be part of it, too. Take a look at David Letterman for another example.
Not to drift too far off topic, but you get to a point where you can only be funny for so long before it all gets stale. Jack Paar, who was probably one of the best observational comedians of all time, left television after eight years, knowing he couldn't sustain himself beyond that.Contrast that with Johnny Carson, who (like Letterman) stayed in the game for 30 years, perhaps a little too long. By the end, Arsenio Hall was beating him. Carson was simply out of tricks; he was nostalgia by that point. Now, that doesn't mean you can't have a career renaissance (Jay Leno is a perfect example) but you have to have a perfect convergence of outside factors for that to work.