2 main cases I would guess:

1) Big press is now interested in YCombinator, and therefore reaches out independently to YCombinator companies

2) YCombinator staff+alum have press relationships from previous businesses/life, and therefore are able to provide a "warm" intro for the startup. If the startup has a good story, the press is receptive.

I can't speak to any other YC company, but I think drchrono certainly stands on its own merits here. Great looking product + hot industry + ongoing major news topic = mind blowingly awesome press.