Glaucoma is often caused by pressure on the optic nerve. However, a feeling of pressure behind the eye is generally only felt with closed-angle glaucoma.
An excessive amount of fluid in the eye, called the aqueous [humor] or a sudden blockage to proper drainage causes a buildup and increased pressure on the optic nerve. Drainage of the aqueous [humor] is through the trabecular meshwork, which is located where the cornea and the iris meet.