Either it's an unfinished shot, or something actually physically flips the truck.
I think it's the second, and here's why - if an explosion occurred underneath the rig, why would it flip? Sure, the force could perhaps catapult it into the air, but most likely, the rig would just explode - you know, fuel and all. The Joker would be barbecued, which isn't Batman's style - remember, no killing, and certainly a truck explosion would cause collateral damage. Batman and Gordon and everyone else want to capture Joker ALIVE.
You can also clearly see the flipped truck in other shots from the trailer, and there's no fire - it's simply on its back, with no evidence of an explosion.
So I'm thinking Bats uses some kind of special tool (no doubt Pod or Tumbler-related) to flip the truck from underneath. Perhaps the Tumbler has some sort of hydraulic flipping mechanism that, much like a cow-catcher on the front of a train, can reach out and fling objects/debris - the thing was designed for combat, after all.