Really big stars keep fusing heavier and heavier elements together until they reach iron. It takes too much energy for the heat and pressure in a star to fuse iron into anything heavier. This means that as the iron core of the star gets bigger and makes more iron, the iron chokes the fusion, causing the star to cool. This happens pretty fast, and when it does, the outer parts of the star suddenly are not being held up by heat anymore: The star is out of hydrostatic equilibrium. The star falls very fast into itself, gaining speed faster than the rising heat and pressure can control.