On the moral origins of PC

http://inductivist.blogspot.com/2008/03/rule-to-live-by-wwho-did-you-read-story.html
I think this is a good stab at the question.
It may decode some of the apparent irrationality and inconsistency of these people.
Of course, they're also keen to oppose anything they might imagine an old-fashioned redneck would support.
To allow one's enemies to define one's positions is to play a weak hand.