Bounciness usually happens when there isn't enough rebound damping to keep the spring from "overshooting" as it extends back from a compression event. And there's a little bump and rebound damping happening nearly all of the time while you're driving, even on "smooth" roads.
Stiff bump damping with soft springs would tend to increase strut shaft loading, but the shaft isn't going to fail in compression. On the rebound side, it takes a stiffer spring to increase strut shaft tensile loading. Soft springs can't get the strut piston velocity up as high against firm rebound damping, which means the extension forces on the shaft would be lower with soft springs.
Struts can and do see considerable amounts of bending, and that combined with a notch/scratch/dent in the shaft might be enough to ultimately result in a fatigue failure from bending. Even that sounds like a stretch, but then again yours is the only strut failure of this sort that I've heard of.
Norm
