Theory: spiraling slipstream circles around the fuselage rearward, striking the side of the vertical stab, resulting in a turn/yaw in that direction.
Where it can be argued is (and I may be completely wrong on this, so feel free to correct me)...the vert stab gives the a/c stability about the vertical axis as we all know. If the tail were pushed/displaced in one direction, the opposing side of the vertical stab is now exposed to more airflow and pushed back to its normal position (positive static stability). This is where "for every action, there is an equal and opposite reaction" comes into play. My point is, I think spiraling slipstream COULD be considered one of these tendencies, but I understand why some would argue that in practicality, it is not.