Those lines are the pattern of the polarizing film of the lcd and shouldn't show up in normal circumstances..I've seen them just on Lcds that had suffered liquid damage and humidity got trapped inside.
Sometimes some moderate dry heat and time help liquid to escape and the pattern disappears on its own.
It might be one day the Lcd suffered water damage ? If now your Mac is in a place with high relative humidity it may be some residues are absorbing water from the air.