I've noticed this happening as well, and the explanation that I've stuck with is that the follow-up posts are posted by readers who recognize a similar article they've previously read that's deserving of a (re)post on HN.
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?).