It is not pure popularity, I suspect it is a combination of clicks/upvotes/comments over time. I know time is a key component in the algorithm. If you don't rise fast, you don't rise. Who is commenting/upvoting is also likely an easy metric to add in. If I have more points, my votes are probably worth more than someone without.

Human curation also exists, but I think that is aimed at removing spam and uplifting YC company posts.