Iceiso, Want to teach your dog to stop pulling on the leash? Go to your nearest pet store and ask for a ‘Halti’. Take you pooch with you so you get the right size. This gizmo fits over their snout and clasps behind their head and your leash connects below their mouth. When your dog gets far enough ahead that he/she runs out of leash it will cause their head to turn to the side. Since dogs like to lead with their snout they quickly learn to identify that feeling of running out of leash and slow down just a little for you. Best training tool I ever bought.
As for as why your dog pulls going south, I couldn’t even wager a good guess.
Phu Cat