I've seen it a few times over the years, I'm sure if we inspected every single polyp in our tanks, we'd probably see it more often. I have no idea what causes it, maybe some sort of genetic issue as the polyps tries to grow/spread. Almost reminds me of an embryo that tries to split, creating twins, yet the seperation isn't complete, creating conjoined twins. It a wild strecth and totally a guess. Most likely a bad guess at that.
Mucho