Final answer:
Hard spots on a brake drum are caused by overheating due to repeated or aggressive braking, which leads to metallurgical changes and affects braking effectiveness.
Step-by-step explanation:
Hard spots on a brake drum are typically caused by excessive heat generated during the braking process. When a vehicle's brakes are applied frequently or aggressively, the brake pads and rotors or drums can become extremely hot. This heat can cause localized areas of the brake drum to overheat and form hard spots. Repeated exposure to high temperatures can cause these spots to become hardened due to metallurgical changes in the drum material, leading to less effective braking and potential brake vibration or noise.