I don't know the specifics of the algorithm that HN uses, but reddit used to factor in 'post velocity'.

Assuming HN does something similar then a relatively small number of quick early upvotes can send something to the front page.

A more mixed response from front page only viewers will send it crashing down.

There are a lot more comments than there are upvotes to the article. The HN algorithm appears to penalize that heavily. There seems to be a cliff at about 40 comments: if you hit 40 comments before 40 upvotes, it falls off the front page.