The Koni shock might be confusing the issue because of the built in dust boot. I pulled a Bilstein so I could demonstrate easier.
This picture demonstrates improper install. Note that the shock mount will not slide down any further on the shaft so you would be able to tighten the top nut up as if everything was correct but it would fail in exactly the same fashion yours has.
First washer properly installed. The washer will not slide down the shaft any further.
Proper install of washers and mount.
Hope that helps.