I've noticed this too. I will be researching a topic elsewhere and then it seems to pop up in HN. Am I just looking for patterns where there are none, or is there some trickery happening where HN tracks those activities and mixes in posts more relevant to my interests with the others?
It may also be that there's some common cause for (1) a topic being on HN's front page and (2) someone researching it elsewhere. E.g., maybe a couple of days ago there was an interesting social-media post related to it, and (1) one person saw it and wrote up something interesting enough to get on HN and (2) another person saw it, started digging around to find out more, and then read HN.
(I am not suggesting that that very specific sequence of events happened in this case. It's just an example of how "I was looking this up for other reasons and then I saw something about it on HN" could genuinely happen more often than by chance, without any need for weird tracking-and-advertising shenanigans.)
HN has a single front page for everyone, so it's a recency illusion. You pay more attention to details and skim over titles you haven't been thinking about.