Well, animals with paws can scratch the screen. Cats and a bird video ("videos for cats" on youtube) are a particular vector on how to get your screen scratched...

I think mine got scratched with some sand on the keyboard and closing the lid

The gap between the surface of the screen and the bottom of the laptop is so tiny that when you close the lid, any dust particle, even so small you can barely see them, gets squashed against the screen. When you leave the laptop charging for a while, static seems to build up on the screen, and it attracts dust particles.

You can tell this is not an incidental thing if you examine the surface of a 2 year old screen. Even if you don't have visible scratches, you can see the contours of the trackpad on the screen, where the gap is a fraction of an inch wider, and the screen stays more pristine.

Well meaning cleaning ladies in the office? I lost big parts of the anti glare this way.