I think it has to do with moderation: some content received multiple abuse reports and was hidden. Then these abuse reports were rejected and the content put back into circulation, which we know forces it back into the communications stream as unread, forcing a notification on the mobile app.
Moving content would also have this effect.
Were you going through an upgrade when this happened? We just went through one (we are hosted) and when we'd update something in our dry-run instance, we'd get a push notification through our app of our current instance that led to nothing new.