In 1860 James Clerk Maxwell argued that the molecules of a gas cannot all share one speed — endless collisions must spread them into a definite statistical law. In 2D that law is a lopsided Rayleigh curve; in 3D the familiar Maxwell-Boltzmann distribution.
Ludwig Boltzmann explained why it appears on its own: equilibrium is simply the macrostate that can be built in the most microscopic ways, and it is so overwhelmingly more probable that a gas drifts into it with nothing steering. No force pushes the curve into shape.
Temperature enters only as a summary of that whole distribution — here, with mass and Boltzmann's constant set to one, the mean kinetic energy per molecule is the temperature. Heat the walls and every part of the curve rescales together; fling one molecule fast and the temperature barely stirs. A molecule has a speed. Only the ensemble has a temperature.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.