Listed in order of what I would suspect based on my experience:
1) Rule out predators - sounds like you did this
2) detritus or other solids like GFO or GAC that land on horizontal surfaces (plates and encrusting). I try to dust mine off with a turkey baster every so often. If it's GFO you may pick up a slight brown coloration as it dies.
3) low Mg - I've consistently seen issues start on montes before any other corals and it can be very sporadic as to location
4) Alk swing or burn but generally this usually happens to acropora first.
5) increase in lighting intensity, like changing bulbs - but it will expel zooxanthellae then die, not just die
Side note there is also sponge, or something like a sponge, that I've had show up only on Montes. I haven't seen it actually kill off an area as it seems to coincide with the monte but will show up as small deep purple blotches.