ipad air 3 screen cracked by itself

My iPad air 3 screen cracked without hit or drop or any external factors.

The day it cracked that afternoon I was using it with apple fitness plus to do exercise. That time everything was still fine. Later evening I opened the screen protect cover saw a hairline. In the beginning I thought was hair on the screen tried to get rid of it. Then I found out the screen cracked! I even didn't know how did it happen?! It's just cracked! Unbelievable! Then I started to check with apple support if I can repair the screen. Turned out my iPad warranty was expired. I don't know if I should repair the screen if it would cost me a new iPad price. I just wonder how could the screen crack by itself? Is it supposed to be apple quality issue? And if apple has this issue why I have to pay for repair fee! I suggest Apple should take a good look of this screen cracked issue.