A 28-year-old man with type 1 diabetes mellitus presents to the emergency department with Kussmaul respirations, fruity-smelling breath, and altered mental status. Blood glucose is 450 mg/dL, arterial pH 7.18, HCO₃⁻ 12 mEq/L, and serum ketones are strongly positive. What is the most common precipitating cause of diabetic ketoacidosis in this patient?