If you've ever been around a big bon fire the gasses around burn to were the heat from the center of the fire reaches to. What makes sense is that the grasses had moisture in them and an outside foreign element was energizing the burning up to a certain specific spot. The heated grasses within range dried out and burned but the grasses burning was not enough to keep the fires spreading to dry out more grass and keep it going. Most everything about this messed up situation can easily point to non existent weapons.