wonder how this happened

If you look at that damage real close, you may come to a slightly different conclusion than expansion.
Check the damage beside and on the roller axis.The nearest damage looks like the keeper pummeled it. But look a little further outboard, both sides. There you will see the spots where the retainer edges bit into the rocker. If it was on the loose keeper side a guy might conclude that this happened after the keeper popped out. But notice both sides are so marked. that leads me to conclude that it actually began before the keeper popped and therefore that the retainer is just too tall, for the rollertip design.I wonder if the arm didnt just press onto the retainer enough to allow the loc to pop out. If this makes sense to you(you are the guy with eyes on it), You might consider a different retainer alltogether with a lower profile or the aformentioned lashcaps.If Im right the shims may not help. Then reassemble without the shaft shims and re-read the geometry.
As always just mho, and yes you are the guy with most luck.