I'm guessing you're asking because of this submission (yours) from a little while ago that's somehow even off the Ask page, not just the front page, despite having a good amount of active discussion.

Ask HN seems to be more sensitive to two things than other discussions:

1. Comment:Score ratio. The ratio of comments to points is 3:1, 69 comments and 23 points as I write this. None of the discussion threads are very deep, but when a discussion has a lot of comments but not a lot of points it tends to get moderated (automatically, not by dang & co.) down faster. This probably relates to the flame war detection, though I've seen that kick in more often when comment threads get very deep and more linear than branching.

2. Flags. When Ask HN submissions get flagged, they move down faster than article submissions.

The Parekh discussion has a high score and a high comment count, so (1) is not applicable to it (more comments than points, but more like 4:3 so not over whatever the threshold may be). I have no idea if it was flagged, but if it was since it had active discussion the mods can turn off flags (as it impacts rankings) on a submission by submission basis.