Similar to the Press Awards dinner when Obama teased Trump over the birther thing (and Trump had that "I'll get you" look), the moment that may have hatched Carlson was his beatdown by Jon Stewart, https://www.youtube.com/watch?v=GooQwKDMqcI

after which he decided "I'll do anything to get back at those guys". He's taken on the WWF bad-guy personae, and let's face it, it'll sell until he gets caught in an impropriety (like O'Reilly) or hurts financially (like Alex Jones).