How does keeping the parking brake applied when brake temperatures are high contribute to the dragging brake phenomenon?
From Airbus:


  Why is it not recommended to leave the parking brake ON with hot
  brakes?
  
  The SOP recommends to set the parking brake brake back to OFF once the
  chocks are in place when the brakes are hot (refer to FCOM for
  temperature values). This is to prevent transmitting heat to the brake
  pistons potentially causing seal degradation, hydraulic fluid
  overheating and generation of a black aggregate, that can reduce the
  piston running clearance and then lead to brake dragging.


(bolded for emphasis)