It is the tendency of the torque applied to the axle while accelerating to twist the springs as they resist the torque. Spring over axle is more prone to wrap since you have a greater moment arm from the center line of the axle than to the spring pack (i.e. the differential is farther away from the frame than the spring are). Adding blocks compounds the problem by separating the axle from the springs farther.

You can actually see it happening, get behind or beside a truck while it is pulling away from a stop light. Watch the rear half of the leaf spring pack twist downward. Stock springs with the heavy overload leaf on the bottom will show this the best, you'll see the overload leaf go way down in back until the tall spring clamp attached to it catches on the top of the main leaf. That serves as an anti-wrap device on the stock springs. One reason not to remove it in an attempt to get more travel out of the rear springs. You will get more travel for a while, until the first hard climb, then your springs will end up in the shape of an "S", the front half will be arched upwards, the rear half downwards. Saw it happen once, took just a few seconds of bad wheel hop to trash the springs. Wheel hop is a side effect of axle wrap, axle winds up against the springs, then if the tire loses traction, it slips, unloads the axle and thus the spring, the wheels hop into the air from the springs unloading, then when the weight is back on the wheels, the wrapping starts again until something lets go.