Nothing weird at all. After I learned horse riding (focus: dressage) I started seeing horse-related stuff everywhere - horse carrying trailers on roads, shops, etc.
The "secret" is not some deep universe related mystery but your perception. Those horse-related things were there all along - I just never paid attention, my mind filtered it without any of it ever reaching my consciousness. Your mind filters out lots and lots of headlines when you scroll through the news or HN, your brain reacts when something is relevant.
That there has to be something relevant even if it's niche once in a while is not exactly shocking when HN goes through several dozens of headlines every single day. The site shows you so many diverse topics, it would be strange if you never have this experience.