I've also heard the main cause is oxidation, or the main cause is UV exposure, or ... ;-)
I'm pretty sure I discovered why I got one only in my right eye. I've been swimming laps for 30 years and have a bad habit of only breathing on my right side.
I was swimming after I found out and realized every time I turned my head up to breath, I was looking directly at the sun. Even with tinted goggles, I could see the outline of my pupil. It was like something out of 2001 Space Odyssey or something, and I'd never paid attention to it before.