Probably people flagging it. I tend to flag stories that are "just news" and don't have a particularly compelling link to standard HN topics, or ones in which HN isn't going to have any particularly interesting angle to add to the conversation unique to the community's strengths, or ones that are coming up a lot, like this one. (I mean, yes, nominally everything is on topic, but there's still boundaries.)

There's a strong attractor basin around discussing the same news as everybody else that's worth pushing HN away from, in general.