Why could malnutrition cause anovulation?

Hormone production: In the case of underweight, there is inadeqaute hormone production which affects egg development in the ovary.

Stress response: Malnutrition, whether due to famine, intestinal disease, anorexia nervosa, or extreme dieting, is physically stressful to the body, and results in high levels of cortisol. High cortisol levels suppress the pituitary hormones that trigger ovulation. This is beneficial in the long run because it lowers birth rates in times of famine, and reduces the number of deaths from starvation.