It is most likely overloading of the wings by pulling too many G's with weight on them. The heavier you are the less G's you should pull. Think of your head as weighing 10 lbs and when you pull 1 G your making your head 10lbs heavier as an example(probably not close to right). So, if your wing is rated at 25,000lbs and you have 5,000lbs in stores on that wing that wing can only hold 5 times what you have stored there. So most likely pulling a 5+G maneuver you will over stress the airframe and loose some parts. Now, I am probably wrong on the math and all that but that is the basic principal I believe. Please, correct me if I am wrong.
Also, keep a close eye on what Paul has to say once his testing is finished.