Without being inside the guy's head, there's no way of really knowing whether or not he was intentionally "coasting" during the regular season. I would think the truth is something similar to what Mangini said last week on First Things First (I think that was the show). He said that for some of these veteran guys who have been around for a while it can be hard to "get up" for every play of every regular season game. It's not that this is a conscious thing or they're intentionally "coasting," it's just that the grind and the "been there done that" feeling suppresses the kind of fire that the younger players sometimes have.
But now it's the playoffs. Now it's win or go home. Now the glory of a championship is tangible. And especially for Suh, this is not a "been there done that" situation at all. And in those moments, the elite get that bonfire burning inside them in a way they didn't have and maybe weren't even aware they didn't have in the regular season.
Don't know if all that's true about Suh or not, but it sounds plausible.