The second, and more common, is a perceived violation of baseball's unwritten rules. There are a lot of these, but they mostly boil down to "don't have too much fun" and are exceedingly stupid. Things that are seen as violations of these unwritten rules include:
- Hitting a home run and watching it go over the wall
- Taking too long to run around the bases after hitting a home run
- Taking too long to go to first base after drawing a walk
- Flipping your bat after a hit
- Stealing a base while you're up by a lot or down by a lot
- Bunting for a hit (trying to tap the ball with your bat rather than really hit it) to break up a no-hitter (a game where a pitcher has not surrendered any hits, a very rare occurrence)
Basically, pitchers are sensitive babies and they will throw a 95 MPH projectile at you if you hurt their feelings by being better than them. Sometimes breaking these rules will result in an exchange of hit-by-pitch events (you hit my guy, whether arbitrarily "justified" or not, so I hit you/your guy). Bizarrely, this is tolerated, but if the hitter decides he wants retaliation in an equally violent way (say, by walking up to the pitcher and punching him), this is considered completely unacceptable. I love baseball, but it's a really dumb sport in some ways.