It all started back in the late 60s, when the St Louis Blues, with fighters like the Plager brothers and Noel Picard, physically pummeled the Flyers, leaving them bloody and beaten. At that time, there was no instigator penalty, both fighters would receive equal penalties, no matter who started it (and even if one of the combatants wouldn't even get to throw a punch). Also, there was no third-man-in penalty, which meant several players could gang up on a single opponent. Witnessing his team get both physically pounded and beaten on the scoreboard, Ed Snider, Flyers owner, told his GM to bring in some tough guys. The GM then drafted Schultz, Kelly, Saleski and traded for a tough defenseman named DuPont. That was the start of the "Broad Street Bullies."
And Snider's strategy worked. The Flyers became a winning team and continued to bring in tough guys like Hoyda, Berube, Holmgren, Wilson, Tocchet, Brown, etc. They were especially successful against the Rangers, because Ranger coach Emile Francis felt hockey was a "gentleman's" game and, for the most part, had no enforcers to defend his slick-skating, but physically softer players.
It is interesting to note that the Islanders, fairly new to the league, adopted the Flyers tough-guy strategy (bringing in Gillies, Nystrom, Howatt and others), and using this strategy, became successful -- much more so than the Rangers.