Anemones split for several reasons. One is reproduction, the other is stress. If the anemone is not getting enough light/food/nutrients it will split in two under the theory that 2 smaller nems have a better chance at survival then 1 big one as the smaller doesn't require as much to sustain itself.
Nems are also extremely picky about water conditions , they prefer a nice stable environment, any fluctuation in water chemistry or environment will reek havoc on a nem as it's internal water willl no longer be equalized to the outside water.
Or it could just be over-feeding - we would really need more details about your set-up to be abel to give you a more exact possibility.