I have no idea, but the question was about "how he was connected" to the people he released the info to. The answer was: he wasn't until he made contact himself.

He must've made a list of journalists he knew of who he thought would be sympathetic to the message he wanted to send. Maybe Greenwald/Poitras weren't at the top of the list, but some quick Googling doesn't reveal anything about anyone else he may have tried to contact before them.