Dams are considered "installations containing dangerous forces" under International Humanitarian Law due to the massive impact of a possible destruction on the civilian population and the environment. Dam failures are comparatively rare, but can cause immense damage and loss of life when they occur.Most of the times floods occur bcoz of dams.
Common causes of dam failure include:

1 Sub-standard construction materials/techniques.
2 Spillway design error (South Fork Dam, near failure of Glen Canyon Dam)
3 Geological in stability caused by changes to water levels during filling or poor surveying.
4 Sliding of a mountain into the reservoir (Vajont Dam – not exactly a dam failure, but caused nearly the entire volume of said reservoir to be displaced and overtop the dam)
5 Poor maintenance, especially of outlet pipes 
6 Extreme inflow Human, computer or design error 
7 Internal erosion, especially in earthen dams
8 Earthquakes