For starters, I think it's important to make the distinction between "non-literal mommy gf" and wanting to fuck your actual mother.
If it's explicitly the latter being posted on r9k, it could very well be just one guy who posts about it non-stop. One thing I noticed in my years of using imageboards, is you get these turbo autists who spam the site talking about the same specific topic over and over again. Like they're incapable of discussing anything but that one thing, whatever it is. Actually these types exist all over the internet and they're annoying as fuck, but on imageboards they can create the illusion that it's a bunch of different users if you're not paying attention. They always samefag in threads and sometimes you'll see entire threads where most of the replies are them replying to their own posts. You can often tell from the writing style, specific vocabulary, the timing of the replies, and certain specific combinations of words.