Why does placing a Moto X on my Macbook turn off the screen?

Whenever I place my Moto X down, face-up, so the center of it is just below and to the left of the fn key on a Macbook Pro Retina 15", the screen goes blank black. When I pick it up, I get the lock screen. I think it may be magnetic, since there are certain locations in that area in which my phone will rotate left or right by itself. Or perhaps it has to do with NFC, because it doesn't happen when I place the phone face-down, and the NFC is located in the back?

I've had similar experiences when stacking two MacBooks just the right way. I think it's the magnets being triggered, and the computer thinking it's been closed.

• https://support.apple.com/en-us/HT203315