I think it is a case of stress (duhhhh)
For me, my coral rtn'd when I handled it too much while I was glueing it to a plug! A day latter, it came off the plug, so I took it out and glued it again!!! The next day, it rtn'd.
I am very sure that mine died from over handling!!! Now I was fortunate enough to see it rtn right infront of my eyes the next morning just before lights-on. I took it out, cut the good pieces and glued them to a differnt plug, and the newly cut pieces survived.
For you guys, something must have changed in your tanks to cause the coral to stress out. It can be some inhabitant inside the tank that was stressing the coral. Or it can simply be the allignment of the stars that night
For your general information, the RTN'ing is very peculiar to witness!!! It is as if the coral is shedding and the coloured "skin" just simply comes off the coral!
Just my 2 cents.