Bad habits. Bad bite, extra spacing , bad habits or porr peridontal condition are the main causea.
Drifting teeth. There are many things that can cause teeth to drift. The most common one is loss of bone support from periodontal (gum) disease. Other causes are loss of teeth with resultant bite collapse, an unstable occlusion, parafunctional habits such as biting on pens, post orthodontic treatment and not wearing a retainer, infections, cysts, and tumors.