Novak faces reality and runs
Jay Rosen has an excellent explication of the Novak-storms-off-the-set incident that does the best job of making the point that it most definitely was the Valerie Plame case that was the reason for his outburst and not anything related to the discussion they were having at the time. Jay points out that the carefully constructed wall that Novak had built to protect him from questioning by his colleagues ("My lawyer said no") was pierced by his column on Monday about the case. From that point on Novak was in an impossible situation. It wasn't until Thursday's outburst that Novak himself realized it.
Where I disagree with Jay is his suggestion that Novak did it on purpose in order to create an excuse for him to further avoid answering the questions. Apparently Novak was warned ahead of time that the questions were coming. So, if he didn't want to answer them he could have simply refused to go on the show in the first place. He might have gotten flack from CNN bigwigs about it, but it wouldn't have been the embarassing spectacle that a walk-off is.
No, I don't think Novak planned this public outburst. What I think happened is that he decided to go ahead with the show knowing that he was going to be grilled. He went ahead because he thought he could handle the questioning.
But then Carville made his comment about Novak having to look tough for the right-wingers and Novak responded with "That's Bullshit!" It was at that moment that Novak knew that he WOULDN'T be able to handle the upcoming grilling. If he was going to snap at Carville over a fairly inoccuous comment like that then what would he say when the REAL questions started flowing?
Novak is a quick thinker. You have to be in his business. In the seconds that followed his outburst Novak self-reflected and realized he was in a no win situation. His "let's just drop it" was an external expression of his inner desire to run away. But then he realized he said that to and that was when that inner desire took control and he ran.
Novak was brought down by his own arrogance.