It's basic physics - hot discs and pads expand.
As they cool they contract, with the result that the handbrake isn't holding as tight any more so, if the car is on a slope, there's a good chance of it rolling away.
Vauxhall made a modification to Vectra handbrakes a year or so ago, and issued a sunvisor sticker reminding you not to depress the button when applying the handbrake.