Iron has the highest nuclear binding energy per nucleon of all the elements (not completely true, but sufficiently accurate in an astronomical context). So, fusion of light elements into iron or something lighter is an exothermic process - you gain energy doing it, allowing the star to function. This is what happens in the last stages of a type II supernova. The core of a massive star in its last moments of life is hot and dense enough to fuse silicon into iron. Just before the supernova explosion, there is an iron ball of about 1.4 solar masses at the centre.
The progenitor of a supernova type Ia is a binary system where a "normal" star loses mass to a compact stellar remnant (a white dwarf). Once the white dwarf has accreted enough mass to be above a limit of 1.4 solar massses, fusion starts again, completely disintegrating the compact object.
A SN Ia completely destroys the white dwarf progenitor in a runaway fusion process.
In a SN II, the pressure on the central iron ball exceeds the degeneracy pressure exerted by the electrons in the iron atoms' electron shell. The Fermi principle in quantum mechanics states that no Fermion (such as an electron) may occupy the same quantum mechanical state as another. The pressure exerted here is so large that the electrons of the iron atoms can no longer obey it and are forced into the nucleus, where they react with the protons to form neutrons.
Why do SN Ia enrich their environment with more iron than SN II? This is not so much a matter of iron production, but about how much of that iron ends up in interstellar space where it can be part of a new generation of stars. In a SN Ia, the progenitor is completely destroyed, scattering all its constituent atoms into its host galaxy. A SN II forms a compact remnant, either a neutron star or a black hole. A lot of the later, heavier fusion products end up not being carried outward in the supernova explosion but become part of the compact remnant.
Note that a lot of the heavy elements scattered from a supernova of the "exploding massive star" result from an abundance of neutrinos escaping the central explosion and reacting with the outer shell of lighter elements that is blasted away.