Coburn sure did nuke his ass in the primary. I guess you could say if he got sh*t kicked that badly by Coburn in a race during which he was originally the front runner, that he may have faltered in the general enough to have lost.
Carson himself had an easy time in the Dem primary (although he did face a statewide officeholder, Carroll Fisher, who would later go to prison, but Fisher got only 8%(!) to Carson's 79%). It's amazing how pristine a candidate he seemed to be, and the Dems probably thought he was the next David Boren.
As it was, Carson still fared well against Coburn. Carson carried the bulk of the eastern & Red River counties along the TX border (Coburn won only a paltry two counties along that border, and one along the eastern border, it also being one of the aforementioned two). Coburn held down Carson's rural advantage and ran up decent margins in the urban areas (which ironically was supposed to be Humphreys' initial advantage -- the difference being that Humphreys would've lost those rural counties to Carson in a landslide). As it was, Coburn won with just 52.8%. Carson got 41% and a Libertarian lady took 6%. In 2010, Coburn carried every county.