Not clear what happened there. Either it was flagged by a couple users (fat fingers on mobile is a problem) or it was the "flamewar detector" gone awry. This "feature" demotes stories when the number of comments exceeds the number of upvotes. Your recourse in this situation is to write dan (hn@ycombinator.com) and ask what happened. Usually if it's an obvious false positive he'll restore it quickly. I wouldn't give up just because of this one.

Edit: HNRankings can sometimes give insight: https://hnrankings.info/39653805/