Ask HN: How come some submissions suddenly decrease in their rank?

Nobody except the site admins knows the details, but from various comments that have been posted here, I believe it is the case that a few factors come into play. The mods may manually adjust the ranking of certain stories for various reasons, and there may also be algorithms that adjust rankings based on things like the ratio of votes to comments (or the inverse), and probably other factors that none of us know about.

When rank reaches double digits, makes it to the first page and then suddenly drops like a rock - this is manual intervention by admins.

Likely due to the bias toward the story.

Why should it be number one? Karma is not particularly high.

And low karma-to-comments count depresses the rank noticeably.

It was rank 1 and then suddenly dropped down to 10. Now it's at 28. And the karma is pretty high for the time of the post existing.

>And low karma-to-comments count depresses the rank noticeably.

Which is unfortunate. Merely having a civil, thoughtful conversation in a thread that doesn't generate enough karma implicitly flags it as low quality.