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Aside from running two different brand/style tires on either end of the car, which is not a great idea for several reasons, your main problem is the following:
With half worn tires on the front and brand new rubber on the rear, you're no doubt exceeding 3mm tread depth differential front to rear. That exceeds BMW's recommended limit and will confuse the traction control system in sweeping turns such as freeway on or off ramps. Mine did exactly the same thing and a persistent shop foreman at my dealer searched long and hard before he found the documentation that explained what was happening. The good news is that, since the rears wear 2x faster than the fronts on a Z4, the problem eventually resolves itself as the rear tire tread depth begins to catch up with the front. IMO though, you should have changed all four tires to totally rid yourself of those horrendous RFTs. But that has nothing to do with the issue you describe.
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