Odd that it got broken whilst in a case though. The eInk screen is generally broken because something pushed on the screen, something hit the screen or the entire device was twisted. The cleaners might have dropped it when moving stuff around on your desk to clean I guess?
eInk screens have a plastic top layer but there are very thin glass layers inside. This means you can end up cracking or damaging an internal layer but still have a completely intact top layer. The diagonal lines are usually an indication that the glass inside has cracked and where they join up is generally where the impact or pressure was.
Great that Kindle CS are replacing your Kindle though!