Liberalism is their God, their religion. Their core beliefs. When those foundations are challenged, they cannot be refuted because they are based on fallacious premises. They can only rationally recant their core beliefs, or they must must defend them knowing deep inside they are at odds with reality.
It sets up a terrible cognitive dissonance that eats away at them from the inside, so instead of re-evaluating their premises, they viciously attack those who challenge them.
It is why conservatives were able to remain sane while a scumbag like Obama did his best to remake the country in his marxist and racist principles, and it is why liberals are becoming unhinged now.
Excellent post (and excellent tagline).