Say if PEY or maybe even PEY & Y together was oversold by 10 seats. Then CX needs to select 10 pax to be moved to J. All of the pax with status in PEY has been moved to J already and they still need to select more pax. And normally pax travelling alone has priority.

I have been on flights with Y & PEY oversold by maybe 20 seats one times, you hear a lot of beeps at boarding gate and a lot of boarding passes facing down. Almost everyone queueing in front of me (status pax at the J boarding line) had a new boarding pass.... Maybe because another flight cancelled?