Microsoft contend that such cracks are caused by User Damage. It is not a defect in the screen. In my case I was also extremely careful in using the device yet the cracked screen spontaneously appeared.
The SB3 also has an inherent design flaw in the way the screen interfaces with the keyboard element when closed. Even slight pressure on the back face of the screen causes flexion and pressure on the top of the screen. The weakest point of the screen are the corners and camera aperture and this is where all the cracks have been happening.
Time for Microsoft to wake up and take responsibility for their poor design.