If I remember correctly the isolation valve(on the 737CL) is electrically controlled and if in "AUTO" with ANY of the following switches in the off position the valve will open: L/H Bleed, R/H Bleed, L/H Pack, R/H Pack.
So all I can think of is that it makes sure your packs are off prior to attempting a cross bleed start, however it does only take one pack to be off to open the isolation valve so I can't really see it as the reason. Sorry
