Generally the cause is Mag being low. Once the tissue recedes a secondary infection with occur causing more damage.
An other cause of this can be basic coral nutrition and elements. I have been using Zeovit additives for years to combat this in my Reef tank.
The Korallen Zucht Nano-Power Kit Kit Includes: Coral Vitalizer, Phol's Xtra, Amino Acids LPS & Sponge Power. Are awesome in what they can do for damaged corals. I am nursing corals back to health now from someone doing the blackout thing on their tank with the Nano Power kit. Not a fan of the whole backout thing as I end up getting peoples corals in my coral hospital systems that need a lot of TLC for months after people doing this.