Yellowing of squash leaves can be due to various reasons, including overwatering, nutrient deficiency, pests, and diseases. Overwatering can cause the leaves to turn yellow and eventually die. Nutrient deficiencies, such as lack of nitrogen, can also cause yellowing of leaves. Pests like squash bugs and cucumber beetles can cause damage to the leaves, leading to yellowing. Diseases like powdery mildew and downy mildew can also cause yellowing of leaves.