Why is WoW going back to level 60?

This is something that is a bit confusing to me. I read that they are going to go from 120 back down to 60 again with the next expansion. It seems, well… Counterproductive. How are they going to level people back who are well over 60 and have it stay balanced??

Well from what I understand, everyone who is 120 will drop to 50 so they will have 10 levels to gain.

As for the why, they want to repackage the game and make it more tight fitting, so to speak. The actual power levels will be just as strong and more abilities and what not will be added as well.