Its because the engine is transversly placed.... there is no axial torque on the chassis to cause the rear to swing...
If it happens in an MR2 it could be uneven weight distribution or if you have an LSD, it may be old and the plates have worn un evenly, which isnt uncommon in all cars really.
Sounds about right, but irrespective of the transverse layout, it is still possible to get the car to swing on a standing start, even without an LSD (albeit requiring more determination!).
Maybe everybody should try this little experiment:
Grab a pen, any will do, and try to push it away from you with one finger on one end of the pen. Try it again with more force (so it moves faster) - notice how it swings around, pivoting on the front-end of the pen.
Now, clearly, this is flawed since you're not always pushing parallel to the pen, but it is still demonstrative. It works better if the surface interaction is a bit 'sticky'
What we have, then, is an unstable system resulting from the rear wheels pushing through the chassis to the front wheels (which are resisting forward movement). Hence the rear-end acts like a ball on the sat atop a curved surface - which way will it roll?
OK, if you push the ball a little bit in one direction, it's most likely to fall that way, hence why it may seem that cars always pivot in one direction. But then again, I'd imagine that the particular phenomenon mentioned (chassis twist?) is dependant on the direction of crank rotation...
I can only comment on GT up to and including GT5P, which I have only played with a Sixaxis controller thus far. Anyway, the Tuscan with "normal" tyres (don't remember which precisely, probably the slippiest, knowing me...) and Pro physics did exhibit some interesting momentum swings that I never came across in GT4, but were reminiscent of those found in other games / sims (say, GPL). I didn't really try any standing starts properly, so can't comment, but I imagine that if you impart any kind of sideways movement into the rear-end when the wheels are spinning, it will pivot around. Maybe the physics engine isn't yet able to initiate this sideways momentum, perhaps due to separate handling of lateral and longitudinal slip on the tyres. Coupling these is no trivial matter and requires experience and extensive testing.
Just to let you know. I'm the head tuner for the Ferrari racing team.
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