What can cause one bulging eye?

Possible tumpr. A single bulging eye could be thyroid disease or a tumor behind the eye if this is a new problem. See your ophthalmologist soon.
Many. Some people have asymmetry that is natural. A new bulging eye is not natural and should be medically evaluated. The most common cause of a bulging eye is thyroid disease.
A bulging eye. Needs to be evaluated by an ophthalmologist or a pediatric ophthalmologist, if in a child, asap to find out what is causing increased pressure on or in the globe (eyeball). Start with your primary care doctor today/tomorrow for a medical evaluation & to get a referral straight away.