Why do natural disasters happen?
They happen naturally. But results of them are triggered by human activities.
Natural disasters happen naturally. The critical thing is basically human being triggers their frequencies (by adding more greenhouse gases into the atmosphere) and therefore their results can be magnified.
According to Botkin and Keller (2003) floods, earthquakes, landslides, coastal erosion and frost and freze are natural hazards that are influenced by human.
Why do natural hazards occur? Our planet is a dynamic one. It has been evolving for a very long time. Not surprisingly, it has natural disasters time to time. Human being due to population growth and land use change turns natural hazards into disasters.
Botkin, D. B. and Keller, E. A. (2003). Environmental Science Earth as a Living Planet. (4th Edition).John Wiley and Sons, Inc. Hoboken,New Jersey, USA.