A 58-year-old male factory worker with a 20-year history of chronic obstructive pulmonary disease presents to the emergency department with acute dyspnea and cyanosis. Arterial blood gas analysis reveals: pH 7.28, PaCO₂ 68 mmHg, PaO₂ 42 mmHg, HCO₃⁻ 32 mEq/L, SaO₂ 65%. Chest X-ray shows hyperinflation and bullae. The patient is placed on supplemental oxygen at 6 L/min via nasal cannula. After 2 hours, repeat ABG shows: pH 7.25, PaCO₂ 82 mmHg, PaO₂ 58 mmHg, HCO₃⁻ 36 mEq/L, SaO₂ 88%. Which physiological mechanism best explains the worsening hypercapnia despite improved oxygenation?