A 62-year-old woman with type 2 diabetes mellitus presents with severe hyperglycemia (blood glucose 850 mg/dL), hyperosmolality (320 mOsm/kg), and altered mental status. Arterial pH is 7.32, HCO₃⁻ is 18 mEq/L, and serum ketones are negative. She is on metformin and a sulfonylurea. What is the most common precipitating cause of hyperglycemic hyperosmolar state (HHS) in this patient?