Ask HN: How does ranking work for Tell HN?

I submitted a Tell HN post. It got 80 upvotes in an hour and hit the front page. However, about an hour later the post disappeared from the front page and from pages 2 and 3. It's now only visible in the Ask section.

If the full text of the post is hosted directly on Hacker News, how does the algorithm detect upvote manipulation and hide it from the front page?

People often share a direct link with friends, who then click and upvote. Could the algorithm interpret this kind of organic sharing as manipulation?

Are there any other reasons why a post might disappear from the front page?

My post: https://news.ycombinator.com/item?id=48230104