Cain has scared them, badly, because he presents a real threat to The Annointed One. As such he must be destroyed, much like they tried to destroy Sarah Palin. She scared them too, because when she first arrived on the national scene as McCain's running mate they were scared shitless that it would derail the Obama machine. They have never forgiven her for throwing that fright into them.
If they really thought he didn't have a chance in hell of getting the nomination, as they've been crowing the past few months, he wouldn't even be on their radar except as the butt of an occasional racist joke. You don't see them going after Huntsman or Paul like this.
They are cowardly little Gollum-like creatures who scurry away when you stamp your foot at them, and like all cowards, plot in the dark to get their revenge for your having done so.