I have noticed them pull the plug and set all classes to 9 some two days before departure inspite of the filght being completely booked.

My two cent guess is that SK allows as much overbooking as possible and rather risk issuing DNB Vouchers before they miss good customers who are paying a good fare. Natural selection happens at the gate... mostly for the benefit of the passengers, since many don't mind a bump voucher.

Regards Oliver