- This is a bug / happens randomly; you just noticed it because you were looking (i.e. as you analyse this data); all the posts it's happened to before and since went unnoticed. That's supported by the evidence of your analysis; most of the results don't look any different to other posts.
- It's not the link, but the related activity. Presumably if you're running analysis on HN data, there are a lot of HN requests coming from your machine. Maybe any posts made by your IP are therefore treated as suspect (i.e. the sort of protection you'd expect to avoid automated posting or upvoting... just without that extra sophistication). Perhaps the other posters had something similar... Would be good to see if any of those posts were by the same author; as that may add weight to this theory.
- Other variables... Maybe the algorithm has rules which cause this behaviour under some conditions; e.g. posts made the previous day (not 24 hours ago; but rather before midnight UTC / something like that) lose weight when midnight hits; so posts made moments before suddenly lose enough score to knock them off the top spot; whilst those which had more score before midnight, or were posted just after survive... Many other possibilities such as this may exist; and we'd only know by looking at those variables in the data... What else is common about the posts which are in your post's club vs those which aren't?