What are some reasons for teeth grinding at night? does being overweight contribute somehow? any natural way to stop the involuntary habit?

Maybe: Some studies relate grinding to airway issues and being overweight puts you at increased risk for airway issues. So yes being overweight may contribute to nighttime grinding. See a dentist trained in dealing with breathing issues to address your concerns.

Nocturnal bruxism: More causes that there is space here to record. Yes, obesity can contribute to airway issues/grinding. Please see your Dentist. A simple night guard (occlusal orthotic) may help. Your Dentist will know if further studies and/or referral would be helpful.