Using our great glob of grey matter, it would seem that at the moment of release your arrow goes side to side during its paradoxical travels.
It seems that it is at this point that it is at its wildest movement. IF there was to be any steering made by the broadhead, that would be a perfect time for it to exacerbate the arrows' jump.
A vertically positioned broadhead would look like a large fin, potentially catching wind and doing more movement, while a horizontally positioned broadhead would catch less and maybe steer less. This all occurs before any substantive spin is imparted to the arrow by the fletching.
Of course, it seems that in actuality, it really doesn't happen like this and either way works well. I suggest you try both ways and see what works well for your style.
ChuckC