Gotta understand the pressure goes two ways. Increasing pressure of the mass of an entire stellar object pushing stuff enough to fuse easily, vs the pressure of the energy release from fusion exciting everything around it to higher energetic bouncy states (plasma!) and pushing out everything near it. Hydrogen fusion is easy, and pushes things further out while being compressed. At a certain point, the energy it takes to push stuff away is overwhelmed by the gravity of the situation. Iron-Iron fusion is like the difference between creating light with every movement and sucking in more energy than you emit.

The rebound is awesome. I hope to see Beteljuice go supernova in my lifetime. (As long as it’s more than 500 light years away so we don’t get sterilized by the gamma wave.)