Tempered glass is heated to make it more resistant to direct impact. However, the heating process makes the glass less resistant to shattering if the edge is hit. Thus, if you bump the shower door when opening or closing it, it can cause a fracture on the edge of the door, and the fracture can continually get worse and eventually cause the door to shatter.
Incorrect installation can contribute to spontaneous blow. If the shower door isn't installed correctly, friction between the door and the track it sits on can cause hairline fractures that result in a spontaneous blow. In addition, if a wheel or other part of the track breaks, it can cause the door to slip and therefore shatter.
Tempered glass is meant to resist direct impacts, such as a person falling into the door. However, if the impact is hard enough, the door will shatter anyway. In addition, you might severely injure yourself if you fall into a door. You should never grab onto the shower door to steady yourself if you start to fall while in the shower. Doing so could cause the door to come off the track and possibly shatter.
Tempered glass is meant to lessen the danger of personal injury if the glass shatters. You probably won't need medical attention after coming in contact with shattered tempered glass. However, you should cover the area where glass shatters with a bath towel to prevent other injuries until you can clean up the glass and wash and dress any wounds. Attend to your injuries, and go to the emergency room if you have any concerns about the amount you are bleeding because of a shattered glass door.