Well I had that thought, but chances are that these things are boxed up weeks before the release date starts, so the unanticipated demand wouldn't really effect things. Unless there's some reason that GW knew they couldn't cast up as much as normal for the Tau release... Whether that's supply issues down the line, or maybe they have something else on the go that's taking up production space/time </conspiracy theory butter>
I think Pre-orders might have cut into their plans when they ran out before the release occurred.
Either way Battleforces don't typically drop on release anyways. They tend to come a bit later in the month or the next month normally, if my memory serves me correctly on this.