Sounds like metallurgical impurity due to inclusions...
What I would like to know:
1. Who is the supplier of these springs, and why did that supplier fail to a) maintain purity, and b) detect the departure from the material specification ?
2. If this level of impurity is "normal" within the material specification, why did Toyota not design the spring properly in the beginning?
Finally, is this another example of Toyota's "under-estimating" the frequency and effect of a failure-mode, and/or doing a silent upgrade or repair without informing NHTSA within the 5-day statutory requirement?