I like
to think of it as a jet of water created between the prop tips and the
hull, but I'm not sure if that's an accurate visualization or not.

it is caused by asymetrical thrust of an angled prop shaft. When backing up the
blade coming up to the hull has a much greater "angle of attack" than the blade
going down from the hull. Thus more thrust on one side than the other.

(there is also asym thrust when going forward, but the rudder can compensate
for this as long as the prop is *pushing* water over the rudder, or the boat is
moving forward)

It is also caused to some extent by the contrainment of the prop wash against
the hull on the up side blade, compared to no constrainment on the down side
blade.

the prop will will "walk" in the direction of movement of the bottom blade.