It occurs in a dog because it possesses physical or physiologic traits that predispose it to glaucoma. These are usually predetermined by genetics. Secondary glaucoma means that the disease is secondary to, or caused by, another condition. A common example is a penetrating wound to the eye. Other causes would include bleeding in the eye, inflammation within the eye, attachments or scarring between the iris and lens, and luxation or displacement of the lens.
Lens luxation can cause glaucoma and likewise chronic glaucoma can cause lens luxation.
The pain may present itself with the dog rubbing his eye with his paw, against the furniture or carpet, or your leg. This is a common, and often unnoticed, early sign. Some dogs will also seem to flutter the lids or squint with one eye. The pupil of the affected eye will usually dilate early in the course of the condition. It may still react to a bright light shining in it, but it will do so very slowly.