Why don't surfboards fly out the back of a utility truck when driving on the highway? I have never had it happen myself and every time I throw my boards (shortboards not malibus) in the back of someone's ute, with no cover on the tray, they always assure me they'll be alright( the surfboards) but i never fully believe it so the whole car journey i sit there in constant paranoia  whilst on the highway watching the boards in the rearview mirror, they're shaking and definetly being effected by the air current but they never fly out , so I just sit in suspense in the situation. 
Why don't the surfboards fly out, even at high speed and crosswinds? 
Is it right to call the air current passing around the car mach flow? 
I'd appreciate some help with the theory behind this so i can stop driving around with this fear of my boards being ruined!
 A: A crude model of the UTE is used simulate the pressure field created by the car as it travels through atmospheric air. We must determine the pressure in truck bed because it applies force on the surfboard.  The car travels at a constant $60 ~\text{mph}$ (an arbitrary, worst-case value).
 

UPPER:  The velocity profile of air flowing around the car.
LOWER: The relative (to atmospheric) pressure profile and surface plot.

The position of the surfboard greatly affects your answer.  I assume the surfboard lies flat, entirely within the truck bed (with the tailgate up).  For now, we assume horizontal forces on the surfboard are negligible.  Summation of vertical forces will determine whether the surfboard will fly out.
Pressures act on the upper and lower surfaces of the surfboard.  The relative pressure profile at the bottom of the truck bed (lower surface) is shown below. A similar profile exists along the top of the surfboard. As a simplification, average relative pressures are used to sum forces on the surfboard, where $P_{ave,lower} = .02 \ ~\text{psi}$ and $P_{ave,upper} = .03 \ ~\text{psi}$.


Various surf websites list short-board dimensions and weights, where $A_\text{surfboard} \approx 2100 ~\text{in}^2$ and $W_{surfboard}\approx 6~\text{lbf}$.  At $60 \text{mph}$, the resultant forces due to pressure are:
$$F_{lower} = A_{surfboard}P_{lower} \qquad \text{and} \qquad F_{upper} = A_{surfboard}P_{upper}$$
$$\therefore F_{surfboard} = (F_{lower} - F_{upper}) - W_{surfboard} \approx -27~\text{lbf} $$
Therefore, your surfboard is held down by $\approx 27~\text{lbf}$- Your surfboard apparently testifies to this!  As other answers indicate, there are other considerations.  Accelerations and transient effects are unknown.  While your surfboard appears to be safe at constant speeds, you should remain wary and assume that it could go flying.

This model illustrates two types of fluid flow (Flow Regimes): Laminar flow characterized by 'smooth' profiles, and Turbulent flow characterized by 'chaotic' profiles.  Mach flow exists when a fluid's velocity is greater than its speed of sound ($c_\text{air}\approx 761 ~\text{mph}$), creating a shock wave- it is not relevant in this problem.  *Note that turbulence and road chatter are likely responsible for the surfboards "shifting".
Flow simulations (CFD) are the most accurate way to solve this type of problem, but since you are interested in theory, consider an idealized (less accurate) model of flow: Bernoulli's equation, $\Delta [P + \frac {1}{2}\rho v^2 + \rho g z] = 0$.  It relates pressure and velocity so that an increase in velocity decreases pressure.  This is seen comparing the flow simulation's velocity and pressure profiles, most apparent on the roof where velocity is high (orange) and pressure is low (green).  It is impractical to apply the Bernoulli equation to this problem because neither the velocity or pressure are known as they flow over complex geometry.  Additionally, it assumes incompressible fluids and does not account for turbulence.
A: Sorry but surf boards do fly out. You may not have experienced it personally but keep doing it the way you are and you are bound to experience it.
If you are thinking that as the velocity of the air travels over car it is some how holding the boards in you are incorrect. 
Check out this article 
http://www.gcdataconcepts.com/carairflow.html
"Actually the Bernoulli's Principle states that as fluid velocity increases, static pressure decreases. Static pressure is the pressure felt by an object or person suspended in the fluid and moving with it. It is the pressure felt when air molecules run over the top of your hand with your palm faced down."  
Bumps, sharp turns, and extreme winds can send your board flying.
Its just the sum of the forces acting on your board and yes they can overcome the force of gravity.
Check out some ways to secure your board. Most of these are for pick up trucks but I think you get the picture now.
http://www.thesurfboardman.com/2011/05/fcs-tailgate-surf-racks.html
https://fortress.wa.gov/ecy/publications/publications/0807030.pdf
http://www.surfing-waves.com/forum/viewtopic.php?f=15&t=6812#wrap
"Transporting surfboards is not something to be taken lightly. Aside from the fact that they are expensive and you don't want it to fall out of your truck; they can also become a hazard to other drivers if it flys out.
I have seen countless boards torn to shreds becuase they were not strapped down properly and found themselves lying on the freeway. A pickup truck is especially tricky becuase many assume you can just tie down your board like any other item you would transport in the bed of your truck."
A: The ground effect that is used in the F1 cars make a large pressure differential between the upper and the lower board surfaces. 
If the air can flow freely bellow the surfboard it will fly away, otherwise it is 'glued' to your truck. 
Be precautious and lock it mechanically.
EDIT ADD.
The board behaves like a wing flying near the ground. 
Above it's upper surface the wind flow freely and a lot more air molecules hit the surface (high pressure) than thru the it's lower surface (near the truck surface) because the flux is mostly blocked. 
This aerodynamic ground effect is used to 'glue' the bolides to the ground (without the spoilers, etc, the cars sometimes flew away presenting high visual effects , accidents). 
Details from WP. 

This kind of ground effect is easily illustrated by taking a tarpaulin
  out on a windy day and holding it close to the ground: it can be
  observed that when close enough to the ground the tarp will be drawn
  towards the ground. This is due to Bernoulli's principle; as the tarp
  gets closer to the ground, the cross sectional area available for the
  air passing between it and the ground shrinks. This causes the air to
  accelerate and as a result pressure under the tarp drops while the
  pressure on top is unaffected, and together this results in a net
  downward force. The same principles apply to cars.  

Who knows if the F1 designers were inspired by the surfboards in their trucks.
I prepared a figure to visualize the configuration of the Ground Effect.
 
