This 1 hour and 15 minute video contains an explanation about what causes tempered glass scratches. 
Dan Fields explains exactly what tempered glass defects are and how they occur. He presents magnified images of defective tempered glass, and demonstrates on a poor quality tempered glass sample that it's not the use of a scraper during window cleaning that causes tempered glass scratches. You'll learn the reasons why this scratched glass issue is such a growing problem, as well as how to protect yourself and your business. You'll also learn the importance of educating your clients about poor quality tempered glass.