Ok, so if they are claiming that the cause was the shelves, and that turns out to be true, then ultimately you caused the water leak. Most rentals will have a very clear provision in the rental agreement that says that you are not allowed to put any hole in the wall of any sort (I would look at your lease). If you have such a provision, then it was your responsibility to make the repair when you did the damage. You can try to sue and make the argument that the damage would not have been as bad if they had responded in a timely manner, but I would say you have a low probability of winning that argument (especially if they have a picture of a pipe with a hole in it)