Snoring is noisy rattling sound occurring during sleep. Snoring occurred to obstruction of air in our respiratory track during breathing. When we breathe we take in air through nose or mouth into lungs.
Normally when we are not asleep , our mouth parts like soft palate and uvula keep this tract wide open to allowing air to pass through it. However, excess growth of soft palate causes snoring.

So when these people sleep,these part fall on the back of their throat , obstruction. Hence ,when they inhale air hits these parts making them vibrate,which we hear as snoring.