They are chosen by hand by the CodePen staff or an invited guest just for that purpose. This goes for Pens, Posts, and Collections.
Why do we pick what we pick?
There is no science to it. We just pick things we like. Things we find interesting. Things we find unusual. Things we find beautiful.
Where do we find them?
We having a running search on Twitter that we explore for good new stuff, and we check our mentions on Facebook and Google+. That tends to have good results, because people share things they are proud of.
We also go through Recent Pens to find things, but we're less comprehensive about that.
How can you get yours picked
If we pick your Pen, you'll get a notification in your activity feed. We send the notification immediately when your Pen gets picked, but your Pen will go into a queue of picked Pens and most likely will not appear on the homepage until later in the day. It all depends on how many other Pens are in the queue ahead of yours.
If you aren't picked, it doesn't mean that you don't do good work. We hope you don't take it too personally. Making it to the Picked section might be some temporary fun, but the community you build for yourself here is the real magic.