Adam: Hell or high water we are gonna get him back out! We leave no man behind on MythBusters man.
Adam: Turkey master, I hand you your bird.
Kari: All right! Looks like it's time to pack Buster's bags for the Bahamas. What do you think he'd wear? Shorts or a little thong?
Adam: I... I don't know if Buster has enough actual flesh down there for a thong, but a.
Jamie: He's got no butt at all, in fact he doesn't even have any legs.
Kari: I have a sneaking suspicion that I'm a really, really bad driver.
Adam: I reject your reality, and substitute my own.
Jamie: I think that was one of the most destructive things I've ever done. That was cool.
Jamie: What's the problem, you don't believe the math?
Scottie: Maybe it's a myth that methane is flammable.
Adam: It's not a myth. We're just idiots.
Answer: A more complex and complete answer probably isn't possible here, since we're dealing with fluid dynamics, pressure, boundary levels, etc. A simply answer would the air in the bed is kept there by the fast moving air coming off the roof of the cab. If there was no "secondary bubble" of air, you would have a vacuum in the bed. When air comes off the roof, it needs a place to touch down, which it is able to do with the top of the tailgate. The fast flowing air moving over the bed keeps the air that's already inside the bed from "escaping." However, the air in the bed is still moving and creates a vortex in the bed, which is described as a bubble. When the tailgate is down, the air coming off the roof will gather at the back of the cab and create drag.
Bishop73