I don't think it's fear of pitchforks. They are smug and safe in gated communities, and are not only blissfully certain that the demographics won't bother them...they like the idea of pitchforkers going after white people with children.
I've been around and around with it for many years. There's something about children that liberals just hate, and feel threatened by in some madly irrational and hateful way.
Hate comes from envy, which comes from vanity. Leftism is fundamentally driven by envy. The nature of envy is not to have what someone else has, but to destroy what they have. The stereotypical liberal is too vain and selfish to have a big family but that doesn't stop them from envying the big families of their neighbors. The baby on board signs on cars and the rear window family size stickers really anger them. They believe they can never be truly happy if others have more of anything than them. They secretly work to destroy their neighbors perceived advantages, all the while with a contrived smile on their face. Leftists hate people in general because each person in the world is another person to envy, another obstruction to total equality and Utopian happiness.
The way to counter leftist envy is to point it out publicly at every opportunity and laugh at them. Satan, the vain spirit, and his associates cannot sustain mockery.