From what I have read and heard, it is because of the wet year that we are having in some of the producing states. If the ground is too wet, the wood can't be harvested. That is what "Georgia-Pacific" is saying. Thus with demand high and production low, we get screwed with high prices for a while. The latest hurricane on the East coast didn't help the matter much either for construction grade ply and other forms of wood. The prices should go down after time. I would not stock up on ply now thinking that the prices will continue to rise and not come down--they will as the weather changes. Alter your time frame a little and you should be fine.
Patrick