I'm a respiratory therapist and grinding is highly common with sleep apnea. Basically, the brain stimulates the jaw to try and move in an attempt to open the airway when it's not getting enough oxygen. Losing weight, getting tonsils out (if you still have them), fixing a deviated septum and soft tissue removal (basically, an ENT does a rotor rooter to the upper throat area) or surgery to bring the jaw forward all can help.
60% of the time, losing 30-40lbs will correct it for the majority of people grinding. CPAP helps immensely as well.