I would bump your salinity up a bit, to 1.026 -- though I don't think that had anything to do with what happened.
I would still keep an eye on it, since it is still new, and from your sig your tank is fairly new. But, it could be that something bothered the anemone, or it was just doing an internal water change -- and it does that by expelling all of it water by shrinking down. My haddonis will do that from time to time, and back when I had an LTA, it would do that if I bothered it.
My Haddoni will actually fully disappear under the sand/rock where I can't even tell that it is there.