The Penguins held a 4-1 lead at that point, during the 2nd period Zegras took shot moments after the whistle was blown for offside. The whistle normally means the end of the sequence, but apparently not to Zegras' ears. The sequence clearly displeased Pens' captain, Sidney Crosby who immediately came over to let him know this was a mistake.
Clearly, the message wasn't clear enough since Zegras was back at it when he once again made a play after a whistle and to show his frustration, threw the puck at the other end of the ice.
Zegras is undeniably one of the most skilled players in the game, but his reputation is growing on him. In 53 games he shows a stellar 44 points, but if he keeps running his mouth and disrespecting the game itself, you can bet a seasoned veteran may not be as keen on him as Crosby was.