Like ReefNeck said, some will split because of stress (for a better chance of survival) and others will split because they are healthy.
Also some bta's are more prone to split than others. I don't know what causes this but I have read that the bta's that are collected from the top of the reef (closer to the sunlight) are found in large colonies and split often, while anemones collected from lower in the reef (were less light) don't split often, rely on sexual reproduction, and usually grow large and are solitary.