They can split for many reasons. One could be that they are stressed and splitting for survival purposes, and another could be that they are healthy and splitting to "carry on the family name" so to speak.
I started with 1 RBTA that was about 2" about a year and a half ago, and it split twice so now I have 3 and they are all about 3-4" now.
If your water is fine and they are healthy and eating normally then I wouldnt worry at all. But you might just want to double check your parameters just to be safe.