From The Baseball Codes:
When Phillies rookie Jimmy Rollins flipped his bat after hitting a home run off St. Louis reliever Steve Kline in 2001, the Cardinals pitcher went ballistic, screaming as he followed Rollins around the bases. “I called him every name in the book, tried to get him to fight,” said Kline. The pitcher stopped only upon reaching Philadelphia third baseman Scott Rolen, who was moving into the on-deck circle and alleviated the situation by assuring him that members of the Phillies would take care of it internally.
I bring this up because of Wil Myers’ reaction to the first of two home runs he hit Sunday against Yankees starter Phil Hughes. There’s no mistaking the rookie’s bravado, and the fact that he did it against a seven-year vet struggling to find his way in the game certainly didn’t help matters. (It’s also not the first time for him.)
The Yankees opted against making it a public issue, but place Kline’s commentary after Rollins’ blast—which was only the third of his career—within the mainstream:
“That’s fucking Little League shit. If you’re going to flip the bat, I’m going to flip your helmet next time. You’re a rookie, you respect this game for a while. . . . There’s a code. He should know better than that.”
Kline never responded from the mound, because he faced Rollins only five more times over the course of his career, all with the game on the line. The Yankees visit Tampa Bay in late August. The convictions of New York’s pitching staff will be made apparent then.
The Baseball Codes 
Jeez, pitchers, get over yourselves …
His second flip was a little less dramatic, but he’s gonna get himself drilled, or worse yet, one of his teammates. It’s time for big brother Longo to take the kid aside and tell him to knock it off.
Silly question.. What’s wrong with bat flipping? I just saw Myers do this against the Angels.. To me it looks like he was just tossing his bat and running, but the announcers made a big deal about it. New baseball fan here.
Wayne
The idea is one of showmanship, which transitions quickly into disrespect. The “proper” way to react after hitting a home run involves putting one’s head down and trotting quickly around the bases. (For this, there may be no more venerated player in big league history than Matt Williams.) The bat flip is a look-at-me action, which can be translated by those partial to such things as an in-your-face action. Baseball is far less forgiving of these things, institutionally speaking, than pretty much any other sport.