I also got some sort of partial "status reduction", less than a full ban: my ability to flag posts disappeared. I sent an e-mail and got it reinstated. But I still have no idea what I might have done wrong, and neither does the HN moderator who reinstated me.

I understand that the HN mods don't want to have to explain every action, and when someone really deserves a partial or full ban they don't want to announce it and cause the person to just evade it. I've been in the position of moderating several forums (none quite this big) and I know it's difficult to keep out the trolls and keep false positives low. But the false positive rate on HN seems very high to me. Some of these methods seem to be too widely used on too many good contributors with too little explanation, and as you say, that breaks down the trust the community needs.