Former chess Böhm has a striking explanation for the low-key game by Carlsen. "He was worried that his computer was hacked by the Russians. Whether that is justified or not, I think that's why he avoids the big/main lines (not a good translation, sorry) and the big heavy openings. Instead he takes refuge in the side paths. And that will print its mark on this game. I'm afraid that's the reason. "
Is this analysis directed towards the 12th game only, or the entire match?
My take on it is that even without any fear of hacking, Carlsen would have good reason to respect the massive apparatus of experts who would help Karjakin prepare for the match. Especially their capacity to analyze his own former games, and other games following the lines Carlsen plays most often. That's a good reason to steer away from the main lines. And if we look at how the 12 games have played out, Carlsen has had decent success with this strategy: He has not really been caught by Karjakin's preparations a single time. He has come out of most openings with the better position and with less time spent. It seems to be Karjakin's ability to find good defensive lines over the board that has saved him, not preparation.
That being said, I can see that such a fear as you describe could cause an extra cautious approach to the 12th and final game. Personally, I'm more inclined to think it was due to a tactical move: Use Sunday and Monday to prepare for rapid chess, while Karjakin would spend his time preparing his defense for Monday. Then play for a win only if Karjakin made a mistake early in game 12, otherwise play it safe and take the draw.
I agree with Wingman here, a tactical draw is the most likely explanation for last night's anti-climax. It can be added that Carlsen is not known for extensive opening preparation. He just seeks a playable middlegame where he can accumulate positional advantages. He has actually had decent success with this, but Karjakin has defended tremendously well and Carlsen has perhaps not made the most of some positions.