rudderrudderrat,
the same reason why we turn OFF APU Bleed
just after engine start, or turn ON APU Bleed
just before shutdown (after PARK BRK On):
Turn APU Bleed pb-sw off just after engine start to avoid ingestion of engine exhaust gases. (PRO-NOR-SOP-09), and;
Select APU Bleed ON just before engine shutdown to prevent engine exhaust fumes from entering the air conditioning. (PRO-NOR-SOP-25)
