After multiple conversations with Sean regarding this failure, I thought I should share the prevailing theory as to what failed and why.
In short, it would appear that the steer plate (the purple billet plate) was not flush to the outer ball mount and the steering arm. The wear apparent on the steer plate in the pic below shows wear on the back side of the interface with the steel outer ball tab on the knuckle. This caused deformation of the aluminum steer plate. Placing a straight edge across the plate, there is a visible taper at the wear mark. This progressive deformation caused the loosening of the hardware and the small shock loads probably led to the stud failing in tension at the weakest point (the threads at the top of the taper).
For those that have this kit, I would strongly recommend checking to make sure that you have properly shimmed the tie rod mount. My kit was provided a 3mm shim for the tie rod mount. This should have been more like a 1.5mm shim. The steer plate is machined with a 4mm drop for the outer ball mount to tie rod mount. The actual drop on the Z4M is more like 2.5mm. Using the provided 3mm shim caused my steer plate to sit at an angle and not flush with both mounts.
Pic below shows the actual drop as measured with the steer plate upside down for illustration.
