According to Boswell on Twitter, it was due to "banging on the glass" during the game:
Chris Boswell @WizardOfBoz09
Got kicked out for banging on the glass?! Man that's crazy!! Sorry we just won two fights!2017-6-9 02:43:54
Tensions were running high between the Predators and Penguins on the ice. Penguins star Sidney Crosby shoved Predators defenseman P.K. Subban's head into the ice repeatedly at the end of the first period.
Professional athletes in Pittsburgh have been known to show their passion for the Penguins. In May 2016, during a series between the Penguins and Washington Capitals, Pirates pitcher Gerrit Cole got a little rowdy in the crowd during Game 4.
The Penguins steamrolled the Predators 6-0 in Game 5 to take a 3-2 series lead. Boswell is entering his third season with the Steelers in 2017.