https://www.vice.com/en_us/article/g5yyz3/reddit-moderators-are-protesting-by-changing-their-communities-to-nsfw

In protest of Reddit’s new API fees, the moderators of some big subreddits have cast aside the usual rules and changed their communities to NSFW. 

Reddit doesn't allow ads to run on subreddits that are marked NSFW; in theory, keeping advertisers off of some of the most popular subreddits on the site would deprive the company of some serious ad revenue.