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Because this simulation is nonsense. I don’t mean that pejoratively, just that you can’t get consistent answers from an inconsistent or in this case, under-defined, premise.

If we are talking about negative mass, it may be that mass and inertia become more clearly different things. Something with "negative" mass might be "pushed" by gravity but still retain normal inertia when touching other types of matter. So the two balls would still bounce off each other as normal, but the gravity interaction between the two would become weird and result in a net gravitational force moving both in one direction.

Most of this can be understood with two observations: negative mass has negative kinetic energy and its momentum points opposite the direction of velocity. So if a system composed of negative mass ‘loses’ energy or momentum to the environment it actually gains (negative or absolute) momentum. It’s all very unintuitive of course because it is technically nonsense.

Perhaps because the wall is made from negative mass (?)