This is a common problem with high profile vehicles in the strong cross winds around here. I have seen on several occaisions various SUVs trying to do 70 in these conditions swerve half into a neighboring lane. Its really funny (if it were not so dangerous) to see side-by-side SUV's synchonously (dance) swerve in the winds down the highway. I cannot help but think of the Disney dance of the hippo sugar plumb faries cartoon when I see this!
The worse I had my Prius deflect, and this was in 35 mph cross gusts at 65 mph, was up to the edge of the lane from being centered. I only drove my Prius without the plate for about a month, and it was difficult to tell what the car was doing in these conditions, there was such a caucophony of vibration coming through. The stock Prius is allot less flexible than allot of SUV's out there, apparently. I think I really did not communicate the cross-winds situation here effectively too you, that you would think the Prius is rubbery.
For people used to driving cars, however, such poor highway handling performance as is common in SUV's is a real problem. And as the Prius is a CAR, people expect car performance.
With the plate, the 35 mph side gust caused the Prius to deflect about 6 inches, and with counter steering it could be kept to much less. Due to the reduction in vibration, it was now easy to drive by the seat of one's pants effectively.
The Prius has a slanted rear end, which is essentially two uprights to the rear of the roof, where the opening for the hatch is. The hatch provides little if no structural strength to side loads.
What is probably happening is the tail of the Prius is being twisted by the side loads to the rear roof as its not a full box cross-section there. My guess is it would only take a fraction of an inch of deflection at the rear axle mounts to cause the rear wheel assembly to steer the car off-line.
Toyota in making the car more Sporty, yet Utilitarian a Vehicle, they went over un-trod structural ground (4 door aerodynmically shapped hatchback), most likely. The Prius is a new concept in oh-so-many ways, and I am sure they were concentrating on the HSD first. If they had made the hatch shorter, the plate probably would not have such an effect. But the short hatch people would probably say that doesn't look right. An inside upper brace about 1/3 the way down the hatch would probably help too, but it would also limit cargo bulk.
Similarily, with the super wide and low Corvette, for which Brian at BT Tech learned to design these braces for. This is a similar situation but at the other end of the vehicle spectrum from the Prius. Still, the thin body cross-section of the Corvette makes for lots of rear-end twistiing too, I am sure. Especially with engine torque being transmitted to the rear wheels. Some older Corvette's have the full length hatchback just like the Prius.
The question is not really if the Prius is rubbery, but why do SUV drivers hog the fast lane in cross wind condtions they cannot hope to safely keep up with cars in?