Certain stony corals, like Acropora, and to a lesser extent Montipora and the others, have a tendancy to die extremely rapidly. A large colony can go from fine to bare skeleton overnight. Genrally fraging unaffected protions is the only way to save anything, and it can spread to other corals in the tank. Specific cause is unknown, though stress obviously is a risk factor. In reality it is probably several different things grouped accorting to their speed of spread.
This is different from STN (slow tissue necrosis) where tissue slowly erodes over weeks or months.
Again these only affect stony, SPS corals. Other types die in different ways.. though they dont have nice acronyms.