It is parked at BAW during day, as no space available at SBC and YPR. So with YPR as OS, the shunting decreases between YPR and SBC.
Also with the availability of Green Line, no issues for passengers with YPR as OS.
I do agree that with 23:30 as Dep Time, they could have still continued with SBC as the 3 PFs 8, 9, 10 wont be congested at that time.