Assuming that the problem is the parking brake...
First, don't use the parking brake at all for a few days and see if it continues to happen.
Next, "loosen" the parking brake so that the shoes never engage the inside of the rear rotors. This DIY tells you how to adjust it - but instead loosen it completely.
http://www.pelicanparts.com/techarticles/Boxster_Tech/51-BRAKES-Parking_Brake_Adjustment/51-BRAKES-Parking_Brake_Adjustment.htm
If the problem goes away, then you know that it was your parking brake causing the problem. Next step would be to adjust it properly. Use the above DIY to do so. It may take an iteration or two to get the parking brake set so it doesn't drag and still engages properly when set.
If this process doesn't solve it, you may have a caliper sticking or some other problem.
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