If you look at the motor, the brushes are near the top and the grass is far away at the bottom sitting on top of the mower deck. It is a real stretch to think that any sparks created by the brushes rubbing against the rotor could ignite the grass. The housing is far more likely to catch fire from the heat generated by the motor. Anyways, I think the probability of the scenario that you describe is on the order of being struck by a meteor, possible but extremely unlikely.