And because people love to generate conspiracy theories... they have already stated that it is not the Coopers who have shortened the event. It has nothing to do with the Coopers and has nothing to do with the fireworks festival that will be happening there in mid-August.
It was a decision of the Pennsic staff to do so to allow extra time for the clean up that the SCA volunteer crew has to do at the end of War.
It is not a permanent date chance, rather, they are trying it this year. So people, please be responsible, clean up your site completely and, if possible, try to put in a few hours helping with the general SCA clean-up/pack-out as well.