Looks like a fatigue failure to me.
If it failed due to a single overload, I'd expect the tube to cripple (buckle in) at the bottom. Imagine what would happen if you took a thin aluminim tube and bent it - the tube would squish and form a hinge, but you'd have a hard time tearing it.
Unless it had a weakend starting point such as a normal drill hole with a torn perimeter.
If you're pole is pre-bent, it should be under tension at the top when you don't have the spin up. With the spin up the top goes into compression.
There can only be a compression load if the rig has a solid connection between the pole and the bridle intersection.
In this case, the pole end will move up and down independantly which will cause it to fatigue at the intersection.
How many spi poles have prebend?
