I think that this year it's driven by fed-up conservatives (such as myself) who are refusing to vote for RINO/GOP-e candidates under any circumstances, and who are willing to see the collapse of the Republican party because we don't see it as a credible alternative to the extreme Democrat leftist agenda.
That attitude explains why Obama won the second time. Four million conservatives decided to punish the GOPe and did not vote. That use to be called cutting off the nose to spite the face.