I also noticed the extended hours today. Woo-hoo!
Also note that the MK now has Fire in the Sky scheduled for every day of the week, and they added a mid-week Spectromagic performance.
This isn't necessarily an informed opinion, rather a reasonable guess. Since Disney knows how many advance reservations they have for any given period, obviously they can forecast attendance levels. Reservations must have somewhat exceeded their initial estimates to justify the extended hours.
We last visited in September '02, and they did the same thing back then--extended the MK hours and added Spectromagic / FITS presentations. But at that time I suspected it had more to do with the ticket price increases that had just occurred.
Whatever the case, it's not like the park attendance is going to suddenly double over the historical precedent. When we visited in September, all but a handful of attractions were walk-ons or a 5 min wait, and I'm expecting largely the same thing this time.