I'd like to know what causes this also. I've read that they'll split only when they are happy, and I have also read they'll split when they are unhappy - trying to "survive" hoping one or the other will live. It's probably true a bit in both cases I would guess.
I just noticed my BTA split this week, it's been doing great. I feed it half an uncooked cocktail shrimp a week or every week and a half. It's under 4 T5's in my 75 gallon tank, and loves a lot of flow.
I didn't want it to split - maybe that's the trick. Hope it doesn't split and it probably will.