AJ-A had broke the dam with a precise hit - the dam was starting to break up even as the bomb from AJ-J was released. See documentary video at http://www.youtube.com/watch?v=zi8rh_wiMuo&feature=related
There seems to e confusion around this. This website says that AJ-A created a splash of water but no break. I have seen information saying AJ-A broke the dam a little at the top.