Let us look at some of the reasons for failure of capacitors.
Electrolytic capacitors fail due to leakage or vaporization of the electrolyte inside. This can be caused due to heating in operation. Heating can be caused by either wrong connection or the use of under-rated capacitors.
In electrolytic capacitors heating can cause the formation of gas inside which can explode through the vent provided.
Voltage surges can also cause capacitor failure. Overtime, capacitors re-form themselves to a particular voltage. When an unexpected surge occurs, a failure can take place.
Ceramic capacitors crack during overvoltages. This may create an open or short-circuit.
Tantalum capacitors are specially sensitive to voltage.
Electrolytic and Tantalum capacitors have polarity. The leads are marked positive and negative. Wrong polarity connections of these capacitors can cause explosion or failure.
In addition to these causes, mechanical damage, heat and ageing can also cause capacitor failure.