When you mean "straight up" do you mean laying along the spine of the blade in what some people call a "fencing" or "sabre" grip, or do you mean lifted off the blade so he is only holding the blade with the 4 fingers and not really using the thumb to secure the grip?
Sabre grip is often used because it aids in providing good tip control in use, alot of knives have a thumb ramp designed to rest your thumb there, you don't lose grip strength because the pinch is now front to back rather than side to side. If you mean lifted off the back, that is a bad habit derived from lots of drilling where you capture a limb (usually at the wrist) by pinching it between the back of the blade and the thumb. It can also happen with an "index grip" (index laying along the side of the blade) at long range when the grip is relaxed, the thumb tends to float off when you relax the grip, you are supposed to clamp back down when you strike (just like punching) but often at long range it can float to the side.