By the way things looked in the picture, it looks like it may have been going out for a while. It's possible the needle bearings in the yoke started to loose it's grease, and possible water intrusion further helped with the bearings go out. Finally, metal to metal impact from driving in traffic, stopping and starting, and acceleration over time caused the shaft to break.
Now, after 70k miles having something like this to happen is beyond me, especially on a car that isn't driven hard. I have seen something like this on an x3, but it had 140k miles on it.