It is part of a campaign based on the fact that Reddit doesn't run ads against NSFW content, so high user count subs can mark everything NSFW (whether it is or not) and hurt Reddit in their cashflow.
For some of the subreddits this isn't as far fetched as for others; /r/videos is more of a work to rule to protest given they're now taking swearing very seriously. You have to think of the 14 year old kids that use reddit.
As 14 yo are very rarely at work, NSFW means that the content is for them, right?
Or they understood it mean something larger and, as every 14 yo on earth, they will happily comply and never open content which is marked "please do not open if you are not an adult".
Don't worry, 14 yo won't even get a chance to be exposed to this content - that's what age verification pages are good for, and Internet users are well-known for only ever submitting their actual date of birth on those pages.
This is making me tremendously sad that the writer's strike[1] is blocking new episodes of Last Week Tonight because good lord, that would be AN EPISODE where he covers this.
