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.
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.