What causes leg cramps at night in men?

Many. Not limited to men! usually fatigue. Could be due to medications taken. Diuretics exacerbate this by causiing low potassium, magnesium and perhaps low calcium. Poor hydration contributes to this. If one has poor circulation to the lower extremities (pvd), leg crams can occur. Sometimes we just don't know why they cramp. You may be helped by some meds. See your pcp and discuss.
Don't forget veins. Although there are many potential causes as drs. Proffitt and isobe said, 65% of all varicose vein sufferers have leg cramps. If you have leg cramps and visible veins in your legs, you would be wise to get a check up from a vein specilaist.
Several things. Causes of leg cramps in men/women include tired muscles after prolonged standing or exercise; abnormalities in potassium, calcium, magnesium; dehydration; and venous insufficiency (venous valves become incompetent, leading to leg swelling which contributes to leg cramps).