I don't think anyone knows - at least at our level has hobbyist. I do not mean this disrespectfully towards anyone and speaking of just my opinion especially after owning one for close to 10 years now. We started with one, ended up with two, two because three, three four, and now we are up to 11. This started in a 29 gallon bio cube, which we added another, then merged those into a 40 breeder, and that this past April into a 210 gallon. I've seen a few tanks over run with them and I tend to believe it since I'm heading that way. I couldn't tell you why the first one split or any of them thereafter. I can only confirm that one split due to being damaged recently with the upgrade this past April. I thought I had the rock clear but when I set it down it was resting on one of the RBTA and it split. I'm guessing I pinched it pretty good and that was the only way for it to survive but even then it is just a guess.
I used to feed silver sides. Then heard that wasn't a good idea due to bones. I started to feed chopped shrimp. That was better but later said forget it and just let whatever it is that they do take care of it. The only thing I do know is that some people force splits or propagation but that isn't my cup of tea. Also mine have actually spawned in the tank on a couple occasions now. First time I wasn't sure what caused the cloudy milk looking water. Second time I thought it might have been them. Third time I saw it and it really destroyed the tanks water and pressured the fish and corals until I ran carbon and did a water change. But to your point I couldn't tell you one way or another why they split nor do I think anyone has a doctorate on it or their primary study.