Before we talk about trucks, lets talk about merry-go-rounds. I'm sure that you've ridden on one and felt the relentless "force" trying to pull you off the edge. In your intro physics course they will tell you at some length that there is no such force and that the real force is the one you exert hanging on and that it points inward.
And they are right the experience of the centrifugal pseudo-force is an artifact of trying to do physics in a non-inertial frame., but ...
... with the right mathematical effort you can do physics in a non-inertial frame and the effort yields the set of pseudo-forces that you have to introduce.1
In the case of the truck the pseudo-force you are looking for is $-m\mathbf{\ddot{R}}$ (using the notation of Marion and Thornton). The interpertation of this is that from the point of view of the truck every object in it is subject to a "force" that has magnitude eqaul to its mass times the trucks acceleration (as measured from an inertial frame like, say, the ground) but pointing in the opposite direction from the acceleration.
So when you start moving from a stop the real acceleration points forward, so the pseudo-forces points backward. When you stop the real acceleration vector points backward, so you are "thrown against the seat belt".
Now the interesting thing about these pseudo-forces is that, like gravity, they are proportional to the mass of the object they act on and thus create the same acceleration in all free subjects. In fact we can treat them very much like an extra gravity.2
OK. Now lets talk about the water.
When you start the water is subject to a pseudo-gravity pointing back. I'll try to flow out the back.3 When you stop the water experienced a pseudo-gravity pointing forward and tries to pile up behind the cab.
The truck is badly overloaded, but let's say for arguments sake let's say that you can stop at two tenths of a $g$.5
Water is trying to pile up at the front and the cover is keeping it from doing that. The water develops a pressure just like the water in a pond. Assuming an 8-foot bed the pressure is about 1/8 of an atmosphere right by the cab fading off to zero at the back. Let's just say 1/14 atm averaged over the front half of the bed.4 The bed is also at least 4-feet wide. So 16 square feet at 1 psi. Hmmm ... 144 sq.in/sq.ft ... the force on the cover is more than a ton.
The cover comes off and the water spills around the cab.
1 We don't teach non-inertial frames in the intro course because the math is a pain and it can hide some very important principles we want the students to get.
2 Indeed, in general relativity the gravity you experience on a day to day basis is an inertia pseudo-force.
3 Really (that is as viewed from an inertial frame) it is mostly staying still while the tailgate plow through it, lifting it up and leaving it behind, but remember that we are now trying to do physics in the truck frame.
4 A slightly conservative choice and it makes an easy 1 pound per square inch, which is why I picked that particular figure.
5 For a typical 3/4 ton pickup you've overloaded by a factor somewhere between 3 and 4 so this might even be reasonable.
