I can point to 4 things that led to the turn around.
1. The added depth. Copp is basically great at everything we were collectively bad at. Vatrano has been an incredible value add but I'm very skeptical about him keeping this up for much longer. Its entirely possible that he just needed opportunity, but I doubt it.
2. Miller being elite defensively since the ASB. It may be time to pair him with Fox and see where it goes.
3. Panarin coming back to life. Some of this may be tied to Copp, but this was the single biggest thing that this team needed. We haven't had him and Zibanejad going at the same time all year.
4. Quality of competition. They've stomped on a bunch of really bad teams (I know buffalo has been going pretty well lately) but the only playoff team they've seen since the ASB has been Pittsburgh, who have had their own struggles. Detroit, Ottawa, NJ? These teams suck balls. Yes they would get worked pretty good by the bad teams earlier in the year, but they haven't exactly been asked to beat the leagues best (or really even the leagues "good".)
It is pretty incredible how every team they've played lately (and stomp) ends up being the most disciplined team of all time. This is a trend that I don't like as it goes beyond team control. If the first round opponent is Pittsburgh, I'd expect to be out PP'd by at least a 3:1 ratio. It's been really egregious when the Rangers have played them.
With all of that said, tomorrow's game is a very nice litmus test for this team. It's the first time in 3 weeks that they've had more than a day off before a game (I think) and against the lone divisional opponent who is ahead of you in the standings.