Why does my phone lock when placed near my laptop

[AC Question]

There is a certain spot on my laptop that if i place my Nexus 5 over it the phone will go dark and light up again on the lock screen. Why is this?

 

[hallux]

Many laptops employ a Hall Effect switch or some other magnetic sensor to detect the lid being open or closed. This acting on a sensor in the phone and causing it do what it's doing.

I've had the same thing happen when stacking 2 similar laptops on top of each other, one would ALWAYS turn the display off, I actually thought there was a problem with it.