I am not 100% sure but I think HKG have had problems with computer network for the past 2 years -- terminals with long response times when intercconnect other systems. I read about it in an aviation magazine on JL flight that bandwith problem and other issues slow down booking significantly at many airports. Maybe that is the reason. Because overbooking at CX usually very smooth! Last April they even gave me restaurant voucher, hotel voucher (executive suite room), inflight 50% voucher, and next day F flight on my I class ticket HKG-NRT, and all was ready and prepared at the check-in desk.