During a brief stoppage in play following the incident, fans saw the Nylander high-stick on Pastrnak via a replay on the scoreboard screen and reacted with a series of boo's, which prompted the officials to huddle together. Typically, if an arm does not go up, no penalty is assessed. However, since it's the Leafs, an exception was made and the officials decided to give Nylander a 4-minute double minor, which allowed them to review the play.
Most in Leafs Nation, I'm sure, were confused with the call and expected it to either be reversed or reduced to just 2 minutes, as there was no blood visible anywhere on Pastrnak's face. However, the review took place and the call stood, baffling even Leafs announcer Joe Bowen.
It's things like this that really make my blood boil. The NHL doesn't even try to hide their bias against the Maple Leafs anymore and people seem to be completely okay with it. Think about it -- when was the last time you saw no arm go up on a play, only to have the fans' reactions change the minds of the officials, leading to them assessing a penalty that long after the fact? Unserious league.