i believe its the suspensions extremes in travel - ie fully compressed or extended - which can lead to the dogbone catching or grabbing in the diff cup, which makes them snap.
yea I believe thats the problem too PJ, and this is exaggerated because of the flex in the lower arms, from the shock mount point out to the wheel end flexes a lot on hard landings which in turn move the shaft closer (and beyond) the edge of the diff cup, hence if you have the cup mod, the shaft will bend instead of the cup snapping.