What is wrong with this LCD screen?

My 13inch macbook pro with retina display(late 2012) starts to show a black patch on top right corner last night and then it got worse. There has been no damage or drop to the laptop at all and it all comes out of nowhere. No sign of physical damage. External display works fine. I made a short youtube video of the screen.

https://www.youtube.com/watch?v=sDumat0e...

Fellow experts, does this have be a cracked LCD screen? How can it crack without any visible outside damage. There is no dent at all on the outside. I asked Apple genius and they say it is a crack in the LCD and I need to replace the LCD. I am wondering if there is a chance that display connection went loose inside the machine and that caused this.

Thanks!