A 68-year-old man with long-standing hypertension and COPD presents with exertional dyspnea and palpitations. Examination reveals a persistent wide split S2 and mild bilateral basal crackles. His 12-lead ECG shows the pattern marked **A** in the diagram: QRS duration 140 ms with an rsR complex in V1, wide slurred S waves in leads I, aVL, and V6, and secondary T-wave inversions in V1–V3. Which of the following best explains the mechanism of the ECG findings in this patient?