I know these offers were being made to Economy pax when I flew YYZ-HKG in December. It was CX829 and was packed. They were offering passengers to go on the next flight CX827 and get a free J class upgrade.

I think at certain times of the year like Christmas and Chinese New Year, where peak travel is usually crammed into only a couple of days, overbooking to an extent they have to leave people behind, happens.