Why does Janie speak out on the porch? What drives her to defy Joe?

Janie doesn't speak out until one of the women from town comes to beg for food to feed her children.... the men refuse and make fun of her. Janie comes to her defense. Thus, we can see that Janie was unwilling to speak for herself, but ready and willing to defend the plights of others (Mrs. Robbins/ the mule). Note, Joe does relent and give Mrs. Robbins a small piece of meat.