Why Did Woodward's Source Come Forth?

It's been speculated that Woodward's source must have been cooperating with the prosecuter, because it was the source who told Fitzgerald about their conversation.

Nope. According to the WaPost, Woodward's source had testified earlier. And then, Woodward -- apparently beginning effort to come clean which we hear about now -- approached the source and reminded them about the June 2003 conversation. It was then that the source went to Fitzgerald to tell him about the conversation.

Thus, Woodward's source is presently exposed to obstruction of justice charges, for not having previously mentioned having told Woodward about Plame to the grand jury. And he was looking forward to getting away with it, because Woodward had apparently decided to keep mum.