Because he didn't sign some for some poster's kid on here in the player's parking lot. Don't remember the poster's name.
The thread is somewhere in the Jets Experience forum. It wasn't written in blue, though. You're thinking of "I don't like Sanchez!"
Here's the OP, just for context: A slighted fan (and his kid) created the joke. The story was probably exaggerated for maximum dickishness effect. Even if it wasn't, here's two things fans should remember: 1) An hour after a skull-crushing embarrassment in front of your home crowd is probably not when you're going to catch your favorite player in the best of moods. 2) If every celebrity stopped everything they were doing to acknowledge every fan at every given opportunity, they'd never get a damn thing done. That being said, Sanchez is a dick.
It's a well known fact that sport celebrities who think they have some kind of personal space are dicks.