Snoring could be caused by or related to:

Structural abnormalities

Nasal polyps.   Benign growths typically caused by chronic infections and/or allergies. Usually disappear when cause is treated. Can also be removed in outpatient procedure under anaesthesia. 

Enlarged adenoids (lymphoid  tissue behind the nose)

Being overweight

Excessive fat in neck squeezes throat, making it more likely to collapse during sleep

Allergies / Cold

Allergies and colds causes nasal blockage or narrowing.   Having a blocked nose necessitates breathing through the mouth. In the article referenced below by Dr. A Hodson, most children with blocked nose have their lips slightly separated and look relaxed, and so a blocked nose is often missed by doctors. Adults tend to acquire a certain shape to the mouth which displays the upper front teeth.

Common allergies include:   consuming wheat, milk;  dust;  pets; feathers.  An article by British doctor Dr A H Hodson talks about the connection between a blocked nose / snoring and allergies. He found out unexpectedly that cutting out dietary wheat and milk stopped his storing. (Also that going on the "Stone Age diet", omitting typical allergy-causing foods, caused easy weight loss regardless of calorie intake)

Drinking alcohol / Taking sedatives

Just a couple of drinks can depress the central nervous system (CNS)

•   Causes throat muscles in the back of the throat to relax

•   Promotes congestion by dilating nasal blood vessels.  

 Sleeping on your back

Base of tongue and soft palate rest against the back of the throat, blocking the airway and causing a vibrating sound. The New York Times reported a >2000 person study finding that 54 percent of snorers were "positional snorers”, finding that as a person's BMI increases with their weight, the likelihood of being a positional snorer decreases.

Overtiredness

 When you are overtired throat muscles become more relaxed

Dry air

Sleep Apnea

A likely cause of snoring, if snoring is loud, frequent and combined with excessive daytime sleepiness.    More common in men. Also, people with sleep apnea tend to be overweight

 A person with sleep apnea stops breathing while asleep.    In an attempt to inhale, a suction is created collapsing the windpipe and blocking air flow. Consequently,  blood oxygen levels fall causing the brain to wake the person up, who then snorts or gasps for air and then goes back to sleep and resumes snoring, without being consciously aware of being woken up. This cycle of is repeated possibly hundreds of times throughout the night. By preventing the ability to reach the deepest stages of sleep, this results in daytime drowsiness.

 References

Dr A H Hodson M.A.(Oxon), Why Snoring? Coleford Allergy Clinic Coleford Gloucester. Online article by British Snoring and Sleep Apnea Association