When one rejects morality, one rejects God, the source of all morality. When one rejects God, he becomes like Lucifer: proud, defiant, setting up a parallel empire against God's.
It doesn't necessarily have to start with disbelief in God and follow with the consequences of that. It can occur the other way around, because really what we're saying when we reject morality in general, or in particular (i.e., when we sin), is that we reject God's sovereignty over our lives and/or other human lives.
Today's liberals are on a continuum from the pan-permissiveness of the late 1960's: utter indulgence of pleasure, worship of the self, rejection of authority, opposition to all boundaries, and a hatred for tradition and its values. Along with that comes opposition to any Common Good, because that would interfere with The Self. Today's liberals are just The Me Generation, all "grown up" (sort of) and busy deconstructing destroying whatever is a permanent Good in the world.
And then one is in a position to substitute self as religion, over genuine sanctity, which is always a fruit of humility. Hence, the self-righteousness and sanctimoniousness.