That's because he breaks the rule of announcing his attacks and then Graything might feel that hand suddenly grow hot, then grow tingly before violent pain and destruction is unleashed from Natsu's mouth as a beam of sheer dual energy in the form of fire and Lightning, destonates outward from where he bit down, pushing Gray away only the hand in his mouth remaining trapped within the dragon's jaw who pulls his head back while also planting his feet in Graything's sides and pushing savagely, aiming to also launch Gray like a cannonball all the way to the other side of the room and down some way through the tunnel he just came through.