A big reason has been frustration.
Frustration with the Canadiens' speed, frustration with officiating, frustration with their inability to beat a team that is not nearly as good as they are, at least on paper.
As the second period of Game 2 of their Eastern Conference Second Round series progressed Saturday, that frustration began to rear its ugly head again.
Just as that power play became a standard 5-on-4, Thomas Vanek scored to put the Canadiens ahead 2-1.
The Bruins, led by coach Claude Julien, reached their boiling point and began to show that frustration. They were slow to line up for the faceoff, Jarome Iginla was jawing with Brendan Gallagher, and Julien was called for unsportsmanlike conduct to give the Canadiens a second straight power play.
The game was within reach for the Bruins yet they were getting unhinged.
"The referee," Julien explained afterward, "I kind of told him that I didn't agree with his calls."
In the past, the Bruins have allowed that frustration to be their undoing against the Canadiens.
They didn't let that happen this time.
Even after falling behind 3-1 on another Vanek power-play goal, the Bruins took all the energy they had been using on frustration and channeled it into their game to orchestrate a spectacular comeback with four third-period goals in a 5-3 win that tied the best-of-7 series 1-1 heading to Montreal.
"We had a tough second period, and the start of the third they got that other power-play goal, but the way we just battled back through I felt a lot of [garbage] that we put up with today was pretty indicative of what our team's all about," Julien said. "It just shows that if you focus on the things you need to focus on there's a pretty good team that can accomplish a lot."
Faced with the possibility, or likelihood at that point, of going to Montreal with a 2-0 series deficit, the Bruins found that focus.
Dougie Hamilton's goal at 10:56 of the third through a crowd in front of Montreal goalie Carey Price, who to that point looked invincible, was Boston's second shot of the period. Though the Bruins appeared to leave much of what happened in the second period behind them, they weren't playing particularly well.
Their season, one when they finished with the best record in the NHL, was 10 minutes away from being in serious jeopardy.
But that Hamilton goal changed things dramatically, and the Bruins were rewarded for their renewed concentration. A Patrice Bergeron goal at 14:17 tied the game, and a Reilly Smith goal at 16:28 gave Boston the lead.
"I thought in the second maybe we got a little too charged up or whatever, but we found a way to focus that there in the third and use some of that energy, use the energy in the building and stay as positive as we could," Iginla said. "It felt pretty good to have it go the way it did."
Three goals in 5:32 allowed the Bruins to tie the series and go to Montreal in a much better state of mind, but they are entering an arena where the temptation to allow frustration to overcome them multiplies.
No longer will Iginla and the Bruins be able to use the energy in the building in their favor, with 21,273 screaming fans making Bell Centre a very inhospitable place for a road team, especially one dressed in black and gold.
The Bruins avoided disaster Saturday, but they're not out of the woods.
Uncharacteristic turnovers in their zone and the lack of discipline shown in the first 40 minutes Saturday will need to be left in Boston. The Bruins leave for Montreal on Monday to play Game 3 on Tuesday (7 p.m. ET; NBCSN, CBC, RDS).
But those final 10 minutes of the third period will need to be packed in the Bruins' luggage.
"We've got to be disciplined," forward Shawn Thornton said. "For whatever reason in that building, it always seems to be a couple of things. We know we can't win games if we're killing penalties all night.
"I think that's just common sense."
The Bruins came to their senses just in time Saturday.