What causes someone's eyes to bulge out?

Something behind eye. If your eyes are bulging there could be something behind the eyes causing it. Most common is related to your thyroid but it will need to be evaluated by an ophthalmologist.
Different things . Some people naturally look that way, thyroid disorder can do it. A tumor behind an eye can push it forward, as might blood in the orbit. If you notice a change from previous photos for instance, you may want to see an eye doctor who can use a hertel to measure the amount of "proptosis.".