Unlike the NFL where every possible rule that the league can think of is on the book, there exists a set of unwritten rules in the NHL. In hockey, the law is judged by the players and enforced by, well, 'enforcers' on each team whose job it is to fight when a cheap shot is aimed at your skill players.
Ordinarily fights belong in a ring with gloves, but in certain hockey cases it seems justified. This is one of them.

Lets give this some context. Buffalo Sabres goalkeeper Ryan Miller was playing in his first game back from a head injury sustained from a cheap shot from the Boston Bruins' Milan Lucic. After Miller was injured against Boston, no one on the Sabres went after Lucic for a 1v1 fight or big hit to retaliate. In high school sports retaliation is considered bad sportsmanship, but in the NHL it was seen as a lack of team spirit. Buffalo enforcer/fighter Paul Gaustad stepped up days later to fight Lucic, but by then the rest of the league had already formed a narrative about the Sabres being gutless. That's why the above reaction from the entire Sabres team is justified.
Nashville's Jordin Tootoo made a strong move towards net and probably could not have gotten out of Miller's way at that speed. This play would ordinarily be seen as clumsy and Tootoo probably would have received a few shoves and verbal jabs from the Sabres before entering the penalty box. Because of the circumstance, however, he got four Sabres brush-piled on him throwing punches.
In the unwritten code of hockey, the Sabres came together to show support for its goalkeeper and, by the way, won the game 3-2.