He's just getting started.
That was also largely Tortorella's fault. Hartley had started the fourth line on multiple occasions on the road that season, and lo and behold the only one that resulted in a line brawl was the one where the other coach threw a fit, put on his own fourth line, yelled across the bench, then had a tough defenceman line up at centre for the faceoff in place of a forward. Funny how things work.
In truth, the cause for that line brawl was Tortorella and Hartley's history. At the time, there was really nothing between the players on the two teams that would have them starting that game with a line brawl. It's funny how the players ended up fighting a coach's battle that had nothing to do with them, really.
Anyway, the closest thing we've had to a bench-clearing brawl in the NHL is probably the Penguins/Islanders thing a few years back. In that case, you had a bad hit in the previous encounter, a bunch of minor-league thugs with zero inhibitions brought up, and cheap shots being taken without any subtlety. And the end result was only one incident of a player leaving the bench (to my memory). So I can't really see it happening under any realistic circumstances.