Three causes of coral reef destruction are

too little rain for long periods
higher ocean temperatures
unsound fishing practices
colder water temperatures
pollution from land runoff

1. unsound fishing practices (ie. shock waves from blast fishing, cyanide fishing, fishing nets)
2. pollution from land runoff (ie. harmful chemicals, pesticides)
3. water temperature (corals are more tolerant to warmer temperatures, even reaching 104 degrees Fahrenheit, but cannot survive temperatures below 64 degrees Fahrenheit)

NOTE : I did not see anything about rain levels, but I would assume too low of water levels could be harmful.
Hope this helps!