It's called "pair production." In the cores of the most massive stars, the only thing preventing the core from collapsing is photon radiation pressure, in this case extremely high-energy gamma rays. In some cases, the photon energy reaches levels where it becomes energetically preferable for the photons to decay into massive particles, or rather, a particle and its antiparticle. In this case, an electron and a positron.
The result is that radiation pressure decreases as gamma rays convert into particle pairs. Increasing the core temperature (due to gravitational contraction or from the star being more massive in the first place) simply results in more gamma rays being lost to pair production. Radiation pressure is therefore further decreased.
At some point, the mass of the star can be so high, and pair production so rampant, that the photon radiation flux is no longer sufficient to hold up the core. The core then collapses due to gravity, causing the star to self-destruct in a so-called "pair production" supernova. Only the most massive stars can reach this point; less massive stars may experience some pair production, but due to their lower mass the loss of radiation pressure merely results in core contraction, rather than total collapse. Apparently, if the core collapse releases more energy than the star's gravitational binding energy, the result is just a really loud bang, with no black hole or other degenerate remnant left behind.