[aerohead]

Vorticity is a form of transverse boundary layer contamination, reported very early by NACA (precursor to NASA),during airship hull developments in the 1920s.
All vortices are a result of shear. The shear is a symptom of any adjoining flow fields of varying velocity/pressure (Bernoulli Theorem) which can physically come into contact with one another.
As high pressure always moves towards low pressure, in an effort to reach equilibrium pressure, then we're talking about 'slower' air flowing towards 'faster' air ( again, Bernoulli).
On an automobile, typically, the fastest air (and lowest pressure air) resides on top of the car, as the greenhouse eruption forces it to travel the longest path, while meeting the slowest air at the same time at the rear. ( this is where Julian is dead on,with respect to local pressures).
The only way this can happen is to travel faster over the top.
Since all streamlines are isoenergetic, due to 'conservation of energy' (Thermodynamics), the only way the faster air can gain kinetic energy of motion, is to lose potential energy of local pressure.( It's remarkably like the mass/energy trade-off in Einstein's Theory of Relativity ).
Slower, high pressure air alongside the vehicle is attracted to the low-pressure,fast-moving air above the vehicle, and their velocities where they come together determine whether or not shearing forces will be great enough to spool the intersecting flow up into a vortex ( like the Jet-Stream rolling a rising column of warm,moist,super-cell,Gulf air onto its side to create a tornado).
We know from experiment that, even on simple prismatic bodies, that all vorticity can be 'tuned' out, by careful treatment of the roof angle, side bot-tailing angle, and diffuser angle.
On more rounded shapes the same holds true.
Some longitudinal vortices can be killed simply by properly softening (radiusing) the side/roof/C-Pillar intersection.
What we're looking for is a 'spike-free' pressure environment. Streamline bodies have this architecture. It's why Hucho recommends them. And why I do too.