A 58-year-old man with a 10-year history of poorly controlled hypertension presents with gradual, painless vision loss. Fundoscopy reveals optic disc cupping with a vertical cup-to-disc ratio of 0.8. Intraocular pressure is 28 mmHg in both eyes. Visual field testing shows a superior nasal step defect. What is the most common cause of this visual field defect pattern?