I've wondered about this too. The conspiracy theorist in me came up with some silly ideas. But, in reality my guess is that people associated with YC (alumni, friends, YC co. employees, investors, etc.) vote up posts about launches as part of rooting for & supporting each other. Of course, many people associated w/ YC visit the site so it's almost an automatic front page.

They don't bubble up. They are assigned a high rank to start with and then they fall off the page as per the normal algorithm.