The rear brake discs heat up during use and cool down when you park.
This causes the metal of the discs to expand then contract at a different rate to the pads which are pushed against them by the parking brake.
You'll find this is only a problem when you use your brakes 'enthusiastically'- try to brake more gently and you'll not only eliminate this parking problem, but you'll find that your brakes (and the rest of the car) will last longer.
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