There is a boring story, and there is an interesting story.
The boring story is this: hand and wrist injuries beat the holy hell out of your power. Alex Rodriguez is fresh off just that sort of injury, and as such, he is not hitting at all well precisely when the Yankees need him most. He is not alone in this on the Yankees' roster, but unlike, say, the case of Robinson Cano, the Yankees actually have a backup for A-Rod who can play a little, in the form of the spiderweb-fragile Eric Chavez. And so it makes sense, in an effort to jump-start a legendarily horrific post-season offense, for Joe Girardi to switch out a few parts and maybe see if he can get something rolling. And that's how A-Rod ends up on the bench.
The interesting story is this: A-Rod got pinch-hit for, and because he's such a prima donna and likes having himself painted as a centaur and he's not a team guy, he immediately turned around and passed a baseball to a coupla hot chicks in the stands trying to score some digits rather than do the officially sanctioned stand-on-top-steps-of-dugout-and-look-intense that you're supposed to do when you've been removed from the game. This violation of the unwritten rules of baseball (which sure do get written about a lot, don't they?) has called down the wrath of A-Rod's manager Joe Girardi and his GM Brian Cashman, who, despite ignoring or flat-out denying that Hot Chick Ball Gate (or whatever we're calling it) is the issue, clearly have decided that Hot Chick Ball Gate is the issue. Also, they're going to trade him to the Marlins, who love taking on older players and salary.
One is logical, one is not. One has been flat-out denied by the parties involved, one has not. One is salacious and fits into the ongoing "Whatever A-Rod does is wrong" narrative, one does not.
It doesn't take a genius to figure out which one people are going with. It's just a little sad that the choice wasn't a little harder.