Tobacco? If you smoke, used to smoke, or are exposed to significant second hand smoke, you've got your answer. Alpha 1 antitrypsin deficiency causes emyphysema in non-smokers but is generally inherited. However, you could be an exception.
Smoking and genes. Perhaps you smoke more than your other family members. Perhaps you have a hereditary condition which makes you deficient in an enzyme called alpha1 antitrypsin.