Been doing lots of lunging with my 14yo TW cross. He stops really hard on his front end. Locks his front legs and stiffens up and just drives them in causing him to bounce up in the air a few times. I've never had a horse do this. Usually they just seem to naturally sit deep and stop with their hind end. Worked with him disengaging the hind end and yielding his front end but I'm not sure how to teach him to use his but to stop? Itooks like he could just snap his legs off! Any ideas? Or is this a Tennessee Walker trait?