This apparently happened on our ride Saturday, and at the most inopportune place (top of Hogpen Gap with a 7 mile descent to get home.)
We let the bike fall over on its side at a rest stop. It just fell in the grass, at a complete stop. When we picked it up my stoker said her bars were loose. I tightened them till they felt snug, and we rode on.
Next morning, she told me they were still loose, and inspection revealed the above.
I'm confident they weren't overtorqued. I used a torque wrenc when they were first installed, used carbon assembly paste to lower the need for tightening, and when I tightened them Saturday, I used just a mini tool lightly till snug, not more. Additionally, enough torque to break the stem itself would have likely snapped the ti bolts, or crushed the cf post before trashing the stem.
So the mystery is what happened? Just the bike tiipping over shouldn't have done that. My working hypothesis is that a crash 2 years ago started the problem, and it when unnoticed until it go to the point of failure, and dropping it was the straw that broke the camel's back.
However, there's also the unexplained little gouge on the top of the stem as well, which I never saw before.