A 72-year-old woman with COPD (FEV₁ 35% predicted) and sleep apnea undergoes emergency repair of a perforated peptic ulcer under general anaesthesia. Induction is achieved with etomidate (0.2 mg/kg). Five minutes after intubation, SpO₂ drops to 85% on FiO₂ 1.0, and blood pressure is 92/58 mmHg. Bilateral breath sounds are present. Peak airway pressure is 28 cm H₂O (baseline 18 cm H₂O). Capnograph shows a normal waveform. What is the most likely cause of hypoxia in this patient?