From a link provided elsewhere by Dawg06:
The influx of new members is to offset the rapid loss of Conference USA teams to the Big East, who will add six Conference USA schools over the next two years. That number could increase in the near future, as the Tulsa World has recently published multiple reports that Conference USA member Tulsa is highly likely to jump to the Big East soon. They'd be joined by Southern Mississippi, who have loudly clamored to exit Conference USA since the start of the conference realignment era, and could be given an option to do so by the Big East.
This is the first I've heard about Southern Miss jumping UMass for the second slot.