[Barnsnake]

Here's another way to look at it:
Say you are taking a corner at, or near, the limits of adhesion. If you lift off the throtle, some weight transfers from the rear tires to the front tires (since your center of gravity is above your spindles). Cobras with drivers have a considerable rear axle weight bias. When the weight transfers to the front, the heavier rear loses its grip and you proceed off the road backwards.
911's and other short wheelbase, tail-heavy cars also have this tendency. Running larger tires on the rear helps a bit, but doesn't eliminate the possibility.