Causes of deterioration, and degradation, of coral reefs can be numerous, and many are propagated by man. Let's look at both natural causes and man-made.
It surprises many people to learn that there are natural cause for destruction of coral reefs, but it should not. Coral atolls are basically dead coral reefs from coral that died by being pushed above the water. The earth may seem solid, but it moves over time and as the coral gets shoved up, it deteriorates and forms islands. Even white sands beaches around volcanic islands in the Caribbean is from degrades coral reefs. The volcanoes can have an up-welling or spew out lava that destroys and deteriorates the reefs. all things living have a beginning and and end, and nature has been deteriorating coral reefs for thousands of years! There is also silting and such from rivers that can degrade a reef and break it up quickly.
Man has been destroying the reefs for much less time, but is making up for that by going much quicker and using a variety of ways. They can increase the silting by plowing fields along the coast or rivers that empty into the sea, or by building builds there and having the lots drain dirt into the ocean.
Those fields will also be using pesticides to kill both weeds and insects, as well as worms. The herbicides can kill the algae that is symbiotic and essential for the coral, and the insecticides kill the polyps of the coral. The same applies to the houses and building built near the shore. All this results in coral deterioration, and all could be easily controlled or stopped.
There is also pollution from boats and beach goers. They dump rubbish, spill oil and destroy the oceans, causing massive deterioration of coral reefs.
Then there is global warming, is it natural or human caused? Whatever is the reason, it causes massive deterioration of the coral reefs. The warmer waters causes the coral to die and then the bacteria and such start to work and grow much faster, this deteriorates the coral reefs. There are also stronger storms being spawned, which physically break up the coral reefs.
Between nature and man, the deterioration of the coral reefs is proceeding at the fastest pace ever. We cannot really alter nature, but we can slow down the harm from humans!