Editing before I actually post- this turned out to be incredibly long, convoluted, and rambling. Hopefully it makes a little bit of sense, but I didn't really sit down with an outline organizing my thoughts before I started typing, I just went.
Rflor asked the other day about fighting in hockey, specifically what is commonly referred to as "The Code." First, there's a book I've never read that is called The Code, and is a multiple hundred page book specifically breaking down what I'm going to try to do here in as short a post as possible. Second, as someone said in that thread yesterday, some of this might not make sense unless you've been a fan of the game for a while.
Apparently in the Blues-Hawks game the other day, somebody took a run at Pat Kane, which led to what turned out to be a series of fights. The first was Eager-Crombeen. Rflor didn't understand why a hit on Pat Kane led to these two guys fighting. The most obvious explanation would be if Crombeen was the one who hit Kane. In that case, when a tough guy hits a star player, the tough guy is required by the code to fight a tough guy from the other team. Chicago doesn't have a true heavyweight, so Eager-Crombeen made perfect sense- 2 tough guys of roughly similar size go at it. If Crombeen had done that and then refused to fight someone (see: Avery, Sean), he would be considered a coward who didn't live up to his end of the bargain. A good example of this was a few weeks ago, Toronto vs. Boston. Blake Wheeler of Boston (a big guy but very soft) took a hard run at Phil Kessel (a soft superstar), then refused to fight when challenged by several Leafs. Much disappointment was expressed in the press box by scouts, including Steve Yzerman, who was overheard to mutter "Fight SOMEbody, man."
Again, assuming it was Crombeen who threw the original hit, the other thing he did in that game was make it ok for Chicago's tough guys to take runs at St. Louis's stars. Generally, it is accepted that while any player is fair game for a clean, hard, body check, you're not supposed to go out of your way to finish a check on a star like Pat Kane. (Side note: this is one of the major reasons everyone talks about the increased intensity in the playoffs. Once the playoffs start, it is acceptable to finish any check against any player without retribution) Once one team opens up that can of worms, everyone is fair game. So, for St. Louis, whereas normally guys like Andy McDonald or David Perron or Paul Kariya normally get a little bit of extra space, now the Hawks have a right (some would even say a duty) to even it up by running one of them. You got one of ours, we get one of yours.
Stars who play tough (such as Jarome Iginla, Corey Perry, Milan Lucic, Mike Richards, or to use some classic examples Eric Lindros, Scott Stevens, or Gordie Howe) are a different animal. It's generally accepted that they can hit anyone because they've established themselves as guys who play the game well, but with an edge (as opposed to a mere goon who plays with an edge, but only serves one purpose to his team). They are expected to fight at times, but can turn down fights against lesser players. A great example of this was a few weeks ago in the Flyers-Panthers game. Two weeks prior Mike Richards laid out the Panthers best player, David Booth, with a technically legal but definitely borderline and certainly dangerous hit. Before the game, Florida called up two goons, the massive Steve McIntyre and the undersized but borderline psychotic Mike Duco. The implication was that they were going to be physical and get revenge on the Flyers for the hit. However, Mike Richards is a star player. He would never fight some minor league call-up just to make Florida happy. So instead, Florida's captain and best defenseman Bryan McCabe found Richards early on, they fought, and the score was settled. Richards had to fight a player of (roughly) equal talent, toughness, size, and importance to his team to settle up for what he did. He did not have to fight some meathead.
The meatheads only fight each other, except in very rare circumstances. Most (but not all) teams carry a true heavyweight (usually he's about 6'4, 235-250 pounds, although some guys are bigger and some guys are smaller but crazier). They are generally the nicest and most well-liked guys off the ice. Most of them understand their role is to protect their teammates but intimidating the other team. They are expected to play hard and finish their checks, but rarely do they ever touch another team's star. If a guy whose job is to simply fight took out a guy who could actually play, that would be a violation of the code. That's why you never see Donald Brashear trying to catch Crosby with his head down. When things start getting out of hand physically, the two heavyweights will often find each other in order to calm things down. Growing tension in a game can often only be broken with a fight, and they are the guys to do it. In addition, if the game is getting out of hand on the scoreboard, these guys often find each other. The team that is losing usually sends out their heavyweight in an attempt to get their team fired up. It is generally accepted that the other team's heavyweight owes him a fight because this is how heavyweights demonstrate their value to their teams, and they want to help each other out. With rare exceptions, middleweights are not under an obligation to fight a heavyweight. So if Chicago played Anaheim, Eager (not a true heavyweight, Chicago doesn't carry one) would not have to fight Parros. However, if Anaheim played Toronto, Colton Orr and Parros would be expected to find each other. Circumstances where that would change would be if Eager was running around instigating problems, or if some other debt needed to be settled (say someone else on Chicago took a run at a Ducks star).
Essentially, the natural course of a career separates people into categories- stars, energy guys, and heavyweights. It is generally accepted during the regular season that violence only occurs between people of the same group. Any violation of the Code opens teams up to retribution. Other sports have similar things- if a pitcher drills your clean-up hitter, you hit their clean-up hitter. If he drills your #9 hitter, you drill their #9 hitter. Players who don't follow the Code are looked down upon by teammates, opposition, and management. The Code often seems arbitrary- Mike Richards (tough star) followed the Code by fighting someone after he knocked David Booth (tough star) unconscious with a blindside hit, but Blake Wheeler (soft star) violated the Code by not fighting after hitting Phil Kessel (soft star) once cleanly. Wheeler hit him cleanly and head-on, but is considered a coward for how he followed up the incident. Richards drilled a defenseless player and rendered him unconscious, but is respected for answer the bell later on. Steve Moore (energy player) violated the Code when he cheapshotted Markus Naslund (soft star), then again when he wouldn't fight anyone on the Canucks (until a brief wrestling match with Matt Cooke). Todd Bertuzzi (tough star) violated the Code when he cheapshotted Moore in retaliation for the hit.
It's really hard to explain this without getting incredibly wordy and rambling. Hopefully this helped a bit, but it is really one of those things that eventually makes sense. Most hockey people can call fights five minutes before they happen. I can't count the number of times I hear "Team A owes them one now...here comes a scrap" over the course of a year.