When anemones are under stress, they focus their precious energy on escaping the stress and they divert it to survival which includes expelling zooanthelle, moving and contracting, not reproducing, etc. Could be something wrong here, then it could be normal, what I see from yours dosen't quite seem normal. Least not from any of my BTA's.
I would ask how long you have had the anemone?
What are the tank paramiters now?
What is your lighting?
Do you feed it anything?
Does it have a clownfish?
Is it moving to a new location?
Are you runing a skimmer to remove doc's from the water?
Thomas