It depends on user action. User flags affect submission rankings even before the [flagged] tag appears. There's also a "overheated discussion detector" algorithm that can also depress the rankings of a submission. I think ranking may also be improved depending on the rate of upvotes (besides just quantity), e.g., if a submission gets 4 up-votes in an hour it'll have a higher rank that receiving 4 up-votes over 4 hours.
Mods can add weight to a submission as well, but from what I've seen of their public behavior on the site, I suspect they're not using such weight very often.
>I think ranking may also be improved depending on the rate of upvotes (besides just quantity), e.g., if a submission gets 4 up-votes in an hour it'll have a higher rank that receiving 4 up-votes over 4 hours.
Sure, that's true but go to new submissions page and look at the posts that got 4 votes in 15 minutes. None of them are on the front page let alone in the top 10.
I believe it also has to do with the context of activity of other submissions. Compare the activity of those submissions with the others on the top page.
If you believe someone is pushing an agenda, please be explicit about what you mean. Who is it? What agenda are they pushing? Hand-waving vague accusations sound like vague conspiracies: it's "them"! The nefarious "they"!
Edit to add: From what I've send there is one agenda that is being pushed: keep the community healthy. Try to keep flame wars, off-topic, and other non-constructive discussion to a minimum. If that happens to conflict with what you'd like to see on HN, then likely yes, there is something nefarious going on.
I'm not saying it's nefarious but mods quite often push a story up on the front page with only few votes. I've witnessed it about a dozen times. And given that it happens, they definitely want some stories to be seen. By the definition, that is an agenda -- nefarious or not.
Then there are posts that get into top 10 position with only 4 upvotes in an hour.
Weird. Someone is definitely pushing an agenda.