Why do people wake up when they fart in deep sleep ??

Snap back to reality, frrrrt, there goes gravity

do they? oh good, that means I'm not farting in deep sleep.

Accidental noise. We evolved to be on guard to ensure not attacked in the night. Those who didnt were stinkily eaten and less likely to pass on the jeans they shit.

I've woken up due to the smell once or twice...

Automated shart-verification system.