What causes hypertension?

Many Factors. Hypertension can be secondary due to conditions in cardiovascular system like coarctation of aorta, adrenals or kidneys and renovascular(from renal artery stenosis) or it may be primary also called essential which may be due to genetic predisposition and made worse by life style like obesity, high salt intake, lack of exercise and smoking.
Many things. It could be due to salt in your diet, renal problems, renal artery stenosis, obesity, smoking, pulmonary, problems, cardiac problems, and more.