A 58-year-old woman with HER2-negative, hormone receptor-positive invasive ductal carcinoma of the breast is undergoing chemotherapy with doxorubicin (60 mg/m²) and cyclophosphamide every 3 weeks for 4 cycles. After completing chemotherapy, she presents with fatigue and shortness of breath. An echocardiogram shows a reduced left ventricular ejection fraction (LVEF). What is the most likely cause of her findings?