I've seen exactly that happen before: they zero out a flight well before it's fully booked, and then end up canceling it. I think they do this when they think there's a pretty good chance that they'll cancel it, but aren't quite ready to make it official and permanent. In my case, it was for a flight a week or two out, so the cancellation came within a day or two of the zero-out, but I'd imagine it could be quite a bit longer in the case of a flight that's months away.