Apparently, they really DID get to Holliday, because he turned around and flipped them the bird.
Wow. I don't think major leaguers are supposed to do that. In fact, the official MLB rules expressly forbid it:
"3.09 Players in uniform shall not address or mingle with spectators, nor sit in the stands before, during, or after a game. No manager, coach or player shall address any spectator before or during a game."
Ooh, Holliday could have received disciplinary action for that little gesture.
Holliday may be a little cranky due to the fact that he's slumping, hitting only .267 with 4 home runs, .338 OBP, .412 SLG. He may be suffering from weighty expectations and the criticism that his hitting was inflated by playing at Coors Field. His splits do tell the tale--he hit .357/.423/.645 at Coors and .281/.347/.454 on the road (Baseball Reference). Pretty dramatic disparity.
Hecklers can either be very funny or incredibly annoying. There's nothing worse than a loutish heckler who repeatedly yells the same tired line: "you suck." You've got to be funny and original. It's got to be gratifying to get a reaction from a player. You know you've done your job as a heckler, and helped out the home team by taking a guy out of his game.