Trending subreddits
Reddit adds a list of trending subreddits to its homepage so users can discover new subreddits more easily. They do not specify exactly how the subreddits are chosen to prevent cheating, but say that the trending subreddits are ones that have gained in popularity recently:
Today we’re exploring a new way to show subreddits that have gained in popularity lately by showing you 5 trending subreddits on the front page. Currently, subreddits will be chosen to be on the list based on a Secret Formula™ that updates approximately daily.