How do capacitors fail?
Capacitors are meant to hold electrons on one plate (and repel electrons from the other) while NOT passing a current between the plates.
If current is allowed to pass between the plates than the capacitor has failed.
A capacitor is made up of two isolated plates held apart by a non-conductive material (usually oils). If the oil has been broken down or exposed to a significantly large voltage, electrons will begin to "jump the gap" between the two plates. At this point the capacitor has failed.