I've had many similar events with my BTA's, and I can't really tell for sure why it did it. I've seen this event when I didn't feed them, and yet other BTA's in my tank did not do it at all.
Q's:
Is the anemone near any other type of coral? star polyps or mushrooms for instance?
Post water parameters if possible.
Also I'd like to know where in the tank it is, close to glass, at the bottom..etc..
What type of flow is it getting? turnover rate?
I don't think its anything to worry about. But then again maybe we can find out some common denominator as to why they do it.
Here was one of mine doing the same thing.