I have no proof whatsoever, he could have caught an elbow on a basketball court or a softball on a field. Isn't it just so much more fun to speculate?
Oh, or it could be a bar brawl. Or, he could have anorexia. That seems to be the latest trend. Just giving this topic the gravitas it deserves.
UPDATE: Turns out Mr. Roberts had a "procedure" to remove some basal cell carcinoma spots on his face. It's never as sexy as you want it to be. Via Raw Story