It seems like a combination of HN "top of mind" (ie that HN users submit articles on things they're currently reading/researching), social "top of mind" (ie that people write blog posts on things their peer group is talking about), and a side effect of HN positional age-decay.

If there aren't new and different articles to fill the HN front page, then there has to be something.

And that something ends up being a base layer of blogs about "current stuff".