What Causes Drag:
Wind resistance on the front of a vehicle exponentially increases as vehicle speed doubles, a car for instance pushes roughly 5.5 metric tons of air in front of it for every mile it travels at 65MPH. This high pressure air builds up on the front and flows around and down the sides of the vehicle and is called sheer boundary flow.
As barometric pressure is measured from space to ground the pressure on the base or rear of the vehicle remains relative while in motion to that as when the vehicle was stationary. The quality of a vacuum is measured by how closely it approaches a perfect vacuum, so in this case we will consider that any lower pressure variance constitutes what is considered a vacuum.
As the high pressure sheer boundary flow layers on the sides of the vehicle pass the rear corners of the vehicle they fall or collapse into the low pressure void or vacuum, as these flow layers reattach they start a counter clock wise swirl or vortices in the near wake, or what you recognize as turbulence behind a big truck creating the drag on the vehicle.