Common causes of fatigue during workouts include inadequate hydration, poor nutrition, lack of proper warm-up, and insufficient rest. Dehydration can impair performance and lead to decreased endurance, while a suboptimal diet may not provide the necessary energy for exercise. Lack of a proper warm-up can increase the risk of injury and fatigue. Additionally, inadequate sleep or recovery time can lead to cumulative fatigue, making it essential to address these factors to maintain energy levels and achieve desired fitness results.