Corrosion

There is a plethora of reasons why corrosion occurs, but generally it comes down to the component being exposed to some level of corrosive environment that was unanticipated.  Pragmatically, corrosion often occurs due to reduced maintenance to the area, such as paint or protection being lost.  However, there may be underlying issues to the loss of protection, like cracks forming in paint because of underlying structural cracks at welds.