Pop Warner books up much of All-Star Sports and part of All-Star Music, but the kids spend their days in practice and games, so they don't affect the park crowds much at all.
MouseFest is a total of less than 2,000 people, booking maybe 800-1,000 rooms (out of over 36,000 total), spread all accross WDW property. MouseFest also doesn't affect park crowds much, though you may see roving groups of MFers in the park of the day.
I don't know what's tieing up rooms this year, but I think it's a combination of many factors, starting with travel agents locking up big blocks for conventions, and Disney's room rehab schedule locking out blocks of rooms at various resorts.
It's also possible that the massive changover in the Disney dining reservation system is having some fallout effect, since the dining and room reservation systems are at least connected, though that's probably unlikely. Or they could be making some changes to the room reservation system at the same time that they're changing the dining system.
Whatever the cause of the room shortage, as Cruz91 said, room availability is not directly related to park crowds, so don't panic. I've been to WDW for MouseFest the last 5 years in a row, and the park crowds are ust not that bad. I also expect crowds to be a bit thinner this year due to the recession - YES, Virginia, there IS a recession on, whether you feel the squeeze yourself or not - and although fuel is cheaper now, it was quite high for most of this year and that has eaten up a lot of peoples' vacation budgets.