* Ask HN: Why does a busy man build a shed? (https://news.ycombinator.com/item?id=29767682)
* Why I Live In a Shed (https://news.ycombinator.com/item?id=29762145)
Does HN have some kind of mechanism that promotes posts with similar keywords?
Another easy one to spot: there's sometimes clusters of articles from the same site if it's not a well-known one - people look around the site, find other good stuff and submit that too.
I don't say that this bad or anything, in fact I like that every day has a different topic on the front page. If this is a natural phenomenon then I have not heard about it yet (and it might be interesting to research?).