Skadi had a similar behavior. We used a ball, her favorite thing, to redirect her and have her do obedience before continuing the walk. Our behaviorist told us to incorporate obedience into play... Having the dog sit or lay down, or both, before throwing the ball or frisbee. Stepping on the leash also worked well. What kind of collar/leash/harness is your dog on?
Skadi, AKA Lady Wolfington, GSD 4.6.12
Grim, AKA Bones , GSD 7.7.13