In the December 24, 1979 edition of NY Times, Jim Naughton writes that the brawl began when Rangers goalie John Davidson and Bruins forward Al Secord were having an argument about Secord tripping Ranger Ulf Nilsson.
Secord said in 2009 that he hit Nilsson at the end of the game because Nilsson had "sucker punched" him earlier in the game.
While the argument on the ice was occurring, fans in the nearby sections started to throw food onto the ice and scream at the Bruins players. Around this time, one of the Ranger fans at MSG reached over the glass and punched Bruin Stan Jonathan in the face and grabbed his stick.
When his stick was grabbed, Johnathan, along with Terry O'Reilly, Craig MacTavish, Peter McNab and Mike Milbury climbed over the boards and began to fight with the fans. Eventually the entire Bruins team except for one player was in the crowd.
The fighting lasted for around five minutes before it was broken up by MSG security.
The article notes that around 200-300 "belligerent" people were waiting for the Bruins outside of MSG and rocking their team bus. The Bruins were held inside MSG until security could disperse the crowd.
On the 30th anniversary of the incident in 2009, Dave Seminara wrote in the NY Times that Stan Jonathan was was hit with a rolled up program that drew blood beneath his eyes.
Milbury was reportedly already in the Bruins dressing room when the incident started and then went into "full combat made" before entering the stands.
The end result ended up with every Bruins player being fined $500, except for Gerry Cheevers who was in the locker room, and an eight game suspension for Terry O'Reilly and six games for McNab and Milbury.