Could be a lot of different things. Xenia are really ph sensitive, that would be my first guess. Remember that corals are usually a better indicator of water parameters than most test kits, so you could have some changes that don't register on the kits, but are still there.

also, sometimes xenia just close up for no apparent reason, then open up in a few days. Another thought is if you've added any corals recently, especially sarcophyton, they could be reacting to the presence of new chemicals. These are just guesses.