Comment by k1t

Here's dang's explanation: https://news.ycombinator.com/item?id=26998308

Moderators and a small number of reviewers go through old submissions looking for articles that are in the spirit of the site—gratifying intellectual curiosity—and which seem like they might interest the community. These get put into a hopper from which software randomly picks one every so often and lobs it randomly onto the lower part of the front page.