Iron has the lowest mass per particle of any element . Degeneracy pressure will initially keep the iron core from collapsing. But the iron core will get larger and larger as more iron gets piled on it from the burning of the shells above. Eventually the force of gravity will be great enough to overcome the degeneracy. Electrons get pushed by gravity past the limit of the degeneracy pressure.
Gravity wins again.
When this happens, it happens fast . Collapse halted by neutron degeneracy . The collapse releases an enormous amount of energy. We have a supernova . As the core collapsed, so did the envelope initially.