That's always been a problem with cross-posting.
A newsgroup can live in relative obscurity, but then the moment someone decides to cross-post, it's a pointer to the other newsgroup(s) of the existence of that newsgroup and after that it's fair game. Then the cross-posting continues, as others pick out the "obscure" newsgroup from the existing cross-posting.
People too lazy to look for proper places to post suddenly find more newsgroups because they've been cross-posted to and the see the newsgroup header in some other newsgroup.
In the case of .design it's got to be the current rash of cross-posting that's "moved it up". In our local newsgroup, I'm one of the longest lasting posters, and for a long time my largest percentage of posts were to that newsgroup. Yet that google "profile" for the newsgroup doesn't even list me as a "top ten poster" because in recent years there have been routine cross-posting with other regional newsgroups, and that amounts to far more than the actual local content.
Now, the "regulars" to .design are hardly free of blame for this situation, they continue to replay to the off-topic posts and they basically have had little regard for sticking to the topic in the first place. But I suspect the high traffic is due to the cross-posting.
Michael