dont like trees

Hi i have a 18 month old blue staff whos a great dog,the problem with is when i take him out into the woods each day he starts to strip the bark of trees, now i have had many a bull breed over the years and never as one done that before, just wondering if someone can shed some light why he does..

My last Staff did exactly the same ... he would make a B'line for the biggest tree in sight and do his best to strip as much bark off it as he could before I could get to him the little sh!t bag ... he could just make a start low down, and then tare strips upwards as he pulled against it (sometimes 6-7ft lengths ) it was bloody embarrassing, not to mention probably criminal damage

I put it down to me foolishly throwing sticks for him, and it progressed from there ... lesson learned

Must admit, I've no idea! But then staffs do the strangest things......