A 50-year-old man on ICU is ventilated for ARDS. He has been deeply sedated with propofol for 5 days. The team now wishes to lighten sedation to assess neurological function. After stopping propofol, he develops seizures, hyperthermia, and tachycardia. Propofol infusion syndrome has been excluded (normal CK, no metabolic acidosis). What is the likely cause?