From what CrushBug said it sounds more like their CDN service fubarred. Not much they can do about that besides ensure whatever caused the problem is fixed and re-upload it to the CDN. Certainly nothing they could have done to prevent it. It's not an issue of testing, since everything was fine when the DLC left their system.
Every time you preorder a game, you become part of the problem. Don't be part of the problem.