I think Johnson mishit it and the puck sort of went on end. But Rask, instead of keeping his ground, tried to overplay the far side and moved into the puck.
That's one of the things some goalies like to do. They like to move into the puck as opposed to once they established their position just staying there because they've got the whole net covered. In this case, he figured it was going to the far side so he overplayed the far side, but the puck flipped up and went off the end of his stick.
When the game is that close, any goal against will draw attention, especially with history, but I think Rask was good in Game 2.
On the other side, when you're talking about Brad Marchand, Patrice Bergeron and David Pastrnak, how their game was in Game 2, well it was a struggle. Two of the three goals that beat Rask, that line had the puck and they turned it over.
Marchand passed the puck back to Bergeron, but he passed it behind him. That set up the chance and Ondrej Palat scored to give the Lightning a 3-1 lead.
On Johnson's goal, Pastrnak gave the puck away in the Bruins' attacking zone, Palat flipped it out, Brayden Point chased it down and Johnson scored.
On both giveaways, they're passing the puck back instead of going forward. It almost seems like they're trying to serve Bergeron instead of throwing pucks at the net. They're making back passes and those are getting intercepted, and away the Lightning go.
The Bruins were credited with 13 giveaways in Game 2. Those stats aren't always the best, but 13 giveaways on the final stat sheet is an indication of a problem for the Bruins, especially when six were credited to Pastrnak, Marchand, Charlie McAvoy and Zdeno Chara.