He was already turning toward the crashing sound and reflexively fired the flamethrower, sweeping it back and forth at the ground and trees nearby. It LOOKED like panic firing, or trying to lay down a wall of fire to keep it back, until someone noticed it had burned all the webbing toward the massive spider. His action would let them move freely for about sixty feet around, once the fires died, and he turned back to other direction, doing more sweeping. He waited until the last second to dodge its initial charge, and did so by diving TOWARD it, trying to get under it, where he could barbeque more of it at once.