Here is MY explanation. Might there be another?
A HKG pilot with a bunch of G days is called and asked to fly a long haul. He agrees but it makes his next long haul not legal because he'd not have enough rest after returning from the long haul he's doing on his Gs.
So, crew control grabs a based guy and sends him on the first sector of the long haul originally rostered for the guy who just did the other long haul on his Gs. Now, crew control needs someone to fly back to HKG.
So, crew control grabs a based guy, with reserve before his long haul, and takes him to fly the return sector to HKG. Now, crew control needs someone to replace him.
So, crew control grabs another based guy, with two days of reserve before his long haul, to replace the other based guy who was just called a day early from his rostered long haul.
Anyone have any other explanations?