When you moved the rear end of the leaf spring upwards you tilted the line between the ends of the leaf spring. The leaf spring mostly deforms perpendicular to this line. So by tilting this line you have added a bunch of roll steer. it works like this: When the spring compresses, the center of the spring moves backwards and so twists the axle with the axle moving rearward.
This steers the rear of the car in the direction the axle is now pointing. It is a pretty weird feeling to turn the wheel a certain amount and then as the car rolls outward towards the outside of the curve, the rear end also moves outward. In effect, both ends of the car are steering.

I first noticed this on my '64 D100. When it is heavily loaded it gets pretty unstable going around curves. When going downhill around a curve towing a trailer it becomes a hair-raising experience.

R.