It's only been 5 hours since it was posted, and the story has ~450 upvotes. In contrast, I see another story on the front page right now that was posted 18 hours ago and only has ~250 upvotes.
A big factor of the ranking algorithm is related to the number of comments; as the number increases, the ranking of the article goes down. This is intended to penalize controversial articles that may result in flame wars.
This article has <200 comments, while the other one has >400; this is likely a significant factor in the falling ranking, in addition to the age.
I also believe that upvote velocity is weighted more heavily than upvote quantity, but I could be wrong there.
The comment algorithm seems backwards. I hope HN at least does sentiment analysis to see if the flavor of the comments tends toward being negative before downgrading the post.
