>I think the biggest contributor is just that left-leaning people are more likely to be offended by right-leaning views.

This theory only makes sense if there's an asymmetry between left offended at right and right offended at left. I don't think that's true. At least I don't think its true in the direction of left-more-offended-at-right. A large fraction of right-wing media seems built around selling a feeling of indignation and contempt (Ann Coulter, Bill o', any Murdoch publication etc). I've seen "liberal" used as a sneer by conservatives, but never in the opposite direction.