All Delight....Over the years I have kept up to about 60-70 frags and colonies, and certainly will see this on rare occasion. From what I have seen, some damage or insult occurs to one or more of the polyps and then a bacterial infection (usually in the form of brown Jelly) rapidly goes from polyp to polyp. I cultured this once and did grow a hemolytic strep organism, but cannot be sure at all that was the pathogen. I believe (without any direct proof) that "spoiled" food could also set this in motion. At any rate when I see this happening I soak them in Iodine solution and then frag them (as you did). Generally what I have seen (as you know) is very similar to what we all see with sps, once it starts you better treat and frag quickly...or else.
Sorry for your loss, glad you could save at least one polyp..
MK