What would cause glaucoma?

Many. Primary causes vs. Secondary causes. Genetic predisposition. Anything that damages the eye so that the level of eye pressure is harmful to the optic nerve and causes loss of retinal ganglion cells. The eye drain could be damaged, ocular perfusion could be affected, etc.
Age, heredity, injur. Glaucoma has multiple causes. Most, called open angle, are age related with no actual known cause. Some glaucomas are present at birth and are related to immature anatomy or inherited issues. Others related to disturbance of the anatomy in people who are farsighted. And some are due to trauma. Other conditions also contribute. Sort yours out with your ophthalmologist.