Acropora skin bubbling off.

I've had a small 1.5" frag for about 7 months that I got at the second to the last frag meeting that was doing great. It had doubled in size. Last night I looked at it and it looked like someone had poured paint stripper on it. Its skin was comming off. I syphoned it off and all that was left was the bare white skeleton.  This happened in less than 2 hrs.   Any ideas????

No change in lighting.  I have had the temp fluctuate between 75 & 81 this past week but stable at 78 degrees for 3 days prior to the event.  It was suggested to me that might have caused it. None of the other corals have been affected ie sps, lps, softies etc.