I'm willing to go with quitting or being fired. With the game numbers dropping like they are, heads have to roll. And for Blizz to fire him would make the company look bad, especially after all the proof that he's destroying the game in favor of trying to bring money in by taking in every dumbass with 15 dollars.
Or..
The biggest rat in the company (next to the president of Activision) deserting a sinking ship. You never shit where you sleep and since Ghostcrawler has been shitting all over the place since he took over as lead systems designer, he's run out of room. You can only fuck something up so much before it dies and the numbers (subscriptions) don't lie.
Any other company that loses nearly half of it's profits in less than a third of the time it took to build them up would clean house too. But since Blizz is all about the image, they can't afford to have things looks bad, so Ghostcrawler is "leaving." If you read the wiki on him about his previous job before taking on video games it said that he quit because there was too much departmental politics, filling out paperwork and doing the same thing over and over. I somehow doubt that the position of lead systems designer is any different in those regards.
But no matter what reason - "Ding dong the witch is dead!!!"