Not sure what exactly happened yet, but Joe Girardi and the trainers came out to talk to Andy Pettitte after a pitch and ultimately gave him the hook. He seemed to be stretching out his back after the pitch, and considering his early season back injury, I am speculating that his back was bothering him. Hopefully it's just muscle issues, but I'll update this post when we get more news.
UPDATE (9:51 PM): It's a tight trap, indeed it's the upper back. Thank god.