Why does the collapse of a massive star’s iron core produce a supernova explosion?

Answer: The collapsing iron core rebounds and drives a shock through the star.

The core’s remaining fusion fuel detonates in one runaway flash. — Wrong. This describes a thermonuclear runaway, which can happen on a white dwarf, not in a massive star’s collapsing iron core. A massive star’s fuel does not all detonate at once; the outer layers are blown off by the rebound shock and neutrino heating.

The collapsing iron core rebounds and drives a shock through the star. ✓ — Correct! In a massive star, fusion builds an iron core, and fusion can no longer release energy. The core collapses inward, rebounds, and sends a shock through the infalling layers. Neutrino heating helps revive that shock, blowing the star apart.

A passing black hole’s tides strip the star until its outer layers burst. — Wrong. A black hole’s tides can disrupt a star, but that is not how an iron-core collapse supernova starts. The explosion begins inside the star’s own collapsing core, before any neutron star or black hole is left behind.