What could cause the display to stop working?

Hi

I was watching a movie when display turned off and on couple of times before it died completely. I connected external display to make a timemachine backup. That said macbook itself worked well, its just the display that died. It's 3.1GHz, 16GB of RAM, and 512GB SSD so still keeps high price on second hand market so despite being out of warranty I took it to the genius bar to check it out. After short inspection Apple Specialist said it is water damage, pointed upper right corner, requiring minimum of new logic board, but might be not limited to. So I left it for full inspection and quote. Now, I don't recall any water spill or anything bad happening to my macbook hence my question - what could cause this issue and could something else make a damage that looks like a water ingress?

Thanks,

Pawel