In the game Universe Sandbox, I created a gas giant (made of pure hydrogen) and begun increasing its mass and watched how its radius changed and how it evolved into a star. While it was already a brown dwarf, volume suddenly started to decrease. See the graph below.
At the end of the graph, it became a star and volume started to expand over time with constant mass, which I assume is because fusion became significant and the expansion was due to the radiation pressure. After it stabilized I begun increasing its mass again and before the star became any kind of dwarf I saw that it begun shrinking and then expanding again, until it exploded. See the graph below.
My question is if there is a physical explanation for the sudden collapse of a brown dwarf and the peak in the second graph. I'm assuming that it is due to overcoming some kind of force, just like a star can become more compact when it overcomes electron degeneracy pressure. Unless it is simulated incorrectly, which seems doubtful in this case.
