There are some things that scientists do know for sure, coral diseases act much like human diseases. Think about those times where you might expect that you would get sick...you'd probably be under stress, exposed to unclean conditions, or exposed to others who are sick. The same factors would similarly put corals at risk.
As corals become more stressed by changes in global ocean temperatures and chemistry, introduction of excess nutrients and novel chemicals, or changes in the balance of organisms on the reefs, diseases become more active and deadly. Waste water run-off, fertilizers, deforestation, and other land based activities can make waters ‘unclean’ allowing bacteria or other harmful microbes to flourish, increasing the incidences of coral diseases. And lastly, a reduction in reef diversity leading to species uniformity on reefs means that outbreaks happen more often and are more widespread.