I don't remember where I read this info, but some anemones split in order to survive. Correct me if I'm wrong here. Some leave the other part of the split to die, and move around to have a better living place. If you think about it, it makes sense. The bigger you are, the harder it is for you to move. And the smaller you are, the less food you need to make. That might be the reason behind it. I'm not 100% on this, but I'm sure I read it somewhere. Hope that helps.
Btw, you have two anemones now, right? One IMO is dying, what about the other one?
sebae_the_clown