1) Once you've noticed something, you tend to notice it more often.
2) Novel (or uncommon) articles on HN that get to the front page tend to attract other articles on the same topic. I can imagine this along the lines of:
Human 1: Hey apparently there's an audience on HN for topic X that I enjoy! I wonder if HN is also interested in this related article on topic X.
Or Human 2: Hey here's this interesting related article I discovered while looking at front-page article Y on topic X.
The summary of all of this is to say that my guess is that like attracts like and uncommon topics die down fairly quickly in time. I imagine that it is less of a concerted effort by stakeholders to push and direct topic X than just the effects of a curious and diverse group.