Beyond physical obstruction, sleep apnea can also be neurological which is the more annoying one since only air pressure seems to fix it. You simply stop breathing.

Obstructive apnea doesn't need to be caused by fat either. I always thought sleep apnea was a fat person's problem until I realized at age 30 that when I slept on my back, the tissue mass at the base of my tongue was dropping into my airway. And I'm fit/slim and also a nasal breather.

(I used to think I always woke up gasping for breath in my 20s because I had frequent nightmares. Ugh, wish I could go back and fix it sooner.)