It is only a guess, but my guess is this originated on the LemmyNSFW instance. Ideally they would mark everything NSFW, but some things over there don't get marked, and Lemmy doesn't have an option so the admins can set/flag the whole instance and all content.
This has been noted as a problem before, LemmyNSFW is certainly running the risk of being widely defederated as a result. I think Ernest is suggesting he'll code something into kbin so that anything which originated on that instance can (and will) be treated as NSFW regardless of whether the users over there identified it as such.