As others have also speculated, it's likely to be some auto-moderation system going haywire, either on its own or because of a change in one of the bazillion ot

As others have also speculated, it's likely to be some auto-moderation system going haywire, either on its own or because of a change in one of the bazillion other services below it. It might require creating special one-off tools to crawl through logs and find the affected accounts, so it might take a while, but undoing 99% of the damage as automatically as it occurred should be feasible.