I hear from people all the time of their BTAs splitting and always wandered what I was doing wrong,mine have never split,only to find out it wasn't good when they are splitting
if they feel they are in danger of dying,they will split to double their chances of survival,by them moving off the rock to near the surface,they're either needing more light(they need very strong lighting such as MH or T5)or more water flow or both,they normally stay put when they're happy and they like a place they can get their foot in to feel more secure like in a crevice or hole in the rocks....hopefully,you can correct whatever they're needing and they'll do good for you