Spiraling Slipstream does not have much in the way of a physics
explanation that I can offer in great detail, so here it is explained
Propeller Slipstream (And Its Spiral Tendancy)
The slipstream of the propeller (the air that it pushed toward the tail of the airplane) is not a smooth cylinder of air. Rather, it is a spiral that can actually wrap around the fuselage of the airplane. Some of this spiral ends when it strikes the vertical stabilizer of the airplane:
(Photo: Pilot's Handbook Of Aeronautical Knowledge)
Obviously, this current striking a flat surface creates a notable
force, which, yet again, causes a left yawing moment.