For those of you too lazy to read the report, the conclusion was that the rope was weakened by sulfuric acid (battery acid). They don't know how the acid came in contact with the rope, but it may have been spilled on a parking lot surface (from a car) and picked up when the rope was set down on it.
The moral of the story is keep your ropes clean and watch where you put them. Keeping them in a rope bag is an excellent idea for many reasons.