It's really not. He went up against a weak frontline today. Dwight Powell is terrible on D. Dirk doesn't even try. Like Shoeking said, his motor runs really high. He's very quick and physical.
But he's a turnover machine at the PF spot. Part of this is because he can't finish with his off hand, and always goes back to his dominant hand. Like that lil pump fake over the head thing he does when he drives right.
When a PF sits on it, it's an instant turnover or block.