The pictures on page 25 seem to show how this socket design fails. First because they didn't evenly distribute the wires in the socket (seriously??)... And second, the shear on the zinc from the compressed wire bundle is essentially a narrow rectangle in the direction of the cable with almost all the force concentrated in a narrow forward bit, not really distributed evenly through the socket, because of the wire ends splaying outward and not being equally distributed. I guess the idea is the rest of the zinc towards the back provides a counter force on the front part, but there's a whole lot more zinc in the back pulling on a smaller amount in the front. The tight bundle and the conical shape is great for a tight install, but doesn't help distribution of load. Compare to the Golden Gate Bridge wire termination (https://c7.alamy.com/comp/2RY9PKW/golden-gate-bridge-plans-i...). The zinc creep is definitely bad, but it seems made worse by an uneven distribution of load in the socket, in addition to all the other problems...

tl;dr once the outer wires break in the socket (in part due to zinc creep, but also potentially more load on them), the "glue" keeping the wire bundle connected to the socket is broken, the former wedge keeping the core in place becomes a straight line, and you have a core ready to pop out.