"93 percent of individuals reefs had been affected by a condition known as coral bleaching (which happens when the water is too warm)" ...or when the water is too cold, or when the sun shines too much, or when the corals die off from diseases brought in by ecologists who swim around the area while getting paychecks for goofing around on a boat in the tropics...
As someone who has kept coral in aquariums for several decades, I'm not sure why this post was marked troll. Other than the the part about ecologists causing a bleaching event, it's pretty accurate, though I'm assuming that part was added for humorous effect.
I tended to keep fairly high end systems which had brighter than average lighting. So I've witnessed coral bleaching due to it being kept in dimly lit systems at pet stores and holding facilities and then placed under much brighter lighting.
I've also had heaters that the thermostat became faulty and brought the temperature up to 95F. In my case, conditions were optimal in the tank when this happened and no bleaching occurred. But I've also seen bleaching occur in a couple species in a tank when the temperature hit 90F due to a chiller malfunctioning in the summer.
An individual coral colony can also bleach because a fish or other critter stresses it by picking at it. Nutrient runoff from farming can also cause bleaching, pH changes, etc. Coral is a very sensitive animal and does not do will with sudden changes or changes outside of its very small comfort zone for a lot of parameters. This includes light (brightness and wavelength), nutrient (or even inorganic particulate) content in the water, temperature, pH, water current (velocity and even oscillating vs. constant current. It can also suffocate from lack of current), oxygen content. Some marine invertebrates will only digest food that is of a specific type and size.