ill try and explain and not make it sound stupid with my wording.
Basically, your MAF map has the air that is returned from your BOV automatically calculated in, which also changes depending ont eh amount of boost you make etc etc
when you vent to the atmosphere that air isnt beign returned, so the engine expereices an extremely rich condition from the added fuel it plopped in to compensate for the woosh of air from your BOV.
by using the DECEL air setup on teh SAFC2, you can eliminate the small burst of fuel and be god to go