I'm no expert but I can google. Bit old (2003) but see page 10...
http://www.smartcockpit.com/data/pdf..._Operation.pdf
If a parking brake is applied too soon after a high energy stop, there is a chance that rotors and stators of steel brakes can fuse, or “weld” together. This can result in:
• Inability to move the airplane
• Unscheduled maintenance
• Schedule disruption
• Disruption of airport operations…
Carbon brakes are not susceptible to “welding”