Cause of overlapping toes?

Few reasons: Sometimes the big toe pushes the second toe out of the way simply causes he's stronger, other times certain tendons gain mechanical advantage over others, and sometimes nerve weakness can cause it....

Instability: Toe deformities arise from instability ( or occasionally trauma) of the feet. Faulty biomechanics cause tendons to pull the toes crooked and the joints buckle. Caught early, supportive shoegear and possibly orthotics can help prevent further deformity. Once it has progressed, surgery is usually necessary to correct irritation and pain.