Can only guess it's the manufacturing of the curved glass, the heat of the screen, the pressure on it and that it's brittle because it's G4. It's happened to quite a few edges and will probably show up on the S7 too. Bad news is Samsung doesn't admit to anything and will probably reject the warranty stating you dropped it like I've read so many times. All you can do is call Samsung, or whoever you bought the phone from and try to get it repaired/replaced... for free hopefully.