What causes the airway to close during sleep?

There are several reasons why this would occur.

The upper airway could become blocked due to:

The muscles around your airway relaxing too much during sleep, which blocks sufficient air from getting through. This narrow airway causes a vibration in your throat, which creates the sound of snoring.
The weight of your neck narrowing the airway
Inflamed tonsils, or other temporary reasons
Structural reasons, like the shape of your nose, neck or jaw