I always assumed that those were posts that got flagged too often.

"Spamming", or rather, responding too quickly in an intense discussion, is cause for automatic shadowban here on HN. It happened to me on a previous account some years ago. The posts themselves were harmless, I merely responded to too many users in a too short timeframe. My attempts at having the ban undone also turned out to be a waste of time. Completely absurd.