Understanding the failure mechanism of capacitor
In my experience, overheating tends to cause bulk failure of the metalized film capacitors. As pointed out by Marko above, voltage spikes (thunderstorms, heavy duty appliances switching on and off, switching on and off the equipment itself) produce spikes that can damage those caps over time b y reducing the effective capacitance.
They are called soft-fail because in most applications, they fail open instead of short, so when they are used in EMI filters for instance, that prevents a short circuit across the AC power lines.