Afyon,
Capacitors that are constantly subjected to full mains voltage are prone to failure, it's just something that happens.
When the dielectric insulation between the "plates" of the capacitor fail, you then end up with a dead short between the plates, causing short circuit current to flow and the circuit protection to operate.
If you've ever seen inside a large capacitor, it's like a large roll, that is how you can have such a large capacitance in a physically small component.
I see a lot of capacitor failures with things like single phase air compressors, where the centrifugal switch can jam and leave the start winding and the capacitor in circuit, instead of switch over to the run winding.
