Can't say exactly why this happen, but can tell you what I think could be some of the factors:
1. Space - it seemed to have gone through a growth spurt the last 3-4 weeks. With the toadstool on the left, the xenia on top and the tube below all growing too, things were probably getting a little cramped. From day 1 it has been footed to the same rock, it didn't want to move and down sized.
2. Food - is always abundant in my tank. I feed twice a day, brine/mysis/cyclops. DT's every 2-3 days. Use the enhanced types of brine. Stopped spot feeding the anemone long ago, the clown just throws it all out. The only time the anemone got the bits I gave it, was when the clown threw it out and the tang got it. Clown would steal it back and stuff it in the anemone, so the Tang couldn't get it! lol.
3. Good lighting, the T5s have worked great with this tank.
4. Water - when I test, it's perfect! Weekly water changes, nightly fresh RO top-offs, and wet skimming. I am lazy when it comes to doing all those tests so the only thing tested on a regular basis is the salinity with a refractometer, and PH when I do water changes. Only additive is Kent Coral-Vite.
5. Flow - I change the currents at least once a week. I believe it is healthier for my livestock and helps keep debris from settling in the voids in LR & sand bed.
Both are doing great today. Looks like the new one decided it couldn't get through the xenia field & is moving in the other direction now. The day before this split my daughter was asking if I would get another BTA & have 2 in my 90g. Hehe, I told her no! It must have heard me