What causes the sudden collapse of a when it becomes a supernova?
When the star starts to collapse, its temperature starts increasing and it starts radiating more energy. However, beyond a critical temperature mostly of the radiation is in the form of neutrino particles have no measurable mass or electric charge and hence offer no outward resistance to the inward gravitational force which can thereafter unchecked.
After a supernova fades, is there any evidence of its existence other than a neutron star?
The turbulent expanding cloud of matter thrown out by a supernova can sometimes be detected from Earth after the supernova has faded.