I had some for 4 years, and I noticed they always recessed about this time in summer. My temp was always consistent, so that wasn't it. pH was usually a bit different due to the A/C running, but it was never a huge swing. Since the recession always happened this time of year (and never to any of my other corals), I just always attributed it to some sort of external environmental factor.
So when you say you've seen it happen in the past, do you mean like the past month, or more like a seasonal thing like I mentioned? If it's not a seasonal thing, you may want to track its condition and correlate it to your tank maintenance. Maybe the coral doesn't like your water changes or reacts badly to GFO changes.