OK im no expert on aerodynamics.
But this is what i have experienced.
Any car will shake when standing still and a car or truck passes it at speed. When you pass a semi on the freeway, notice how your car will get pushed in the direction away from the truck just slightly when you are near or just behind the front of the truck. This is the massive amount of wind that the truck is pushing away from its sides as it cuts through the air.
Now cross winds matter more than just how aerodynamic the car is. Suspension geometry, wheelbase, weight, tire size, and the condition of the car's front suspension (most importantly the idler arm) are factors. I just had everything replaced on the front suspension-steering except the tie-rods and my benz was getting pushed around by the wind far more than i expected. MY WRANGLER gets pushed around less. When i tried correcting the steering to compensate for the wind i looked behind at my tail end and noticed how the rear end floated around rather wildly when i did so.
Id have to say with the case of the 123s that its shape and suspension geometry are the greatest culprits to its high wind problems.