A 62-year-old man with hypertension and benign prostatic hyperplasia is started on doxazosin 1 mg at bedtime. Three hours after the first dose, he experiences severe dizziness, near-syncope, and diaphoresis while walking to the bathroom. His wife finds him pale and tremulous. Blood pressure is 85/50 mmHg (supine), heart rate 110 bpm. He recovers after lying down for 30 minutes. Which of the following mechanisms BEST explains this acute adverse effect?