Nice, hang those up on the wall. Hope you weren't hurt when they let go.

I Broke the bars on my mountain bike a number of years ago in the same place (next to the stem). It was a simple bunny hop over a rut and when I came down the bars just snapped, nothing agro but I was going fast enough that I ended up in a gully and taco'd the front wheel as well.

I was at a bike shop where Keith Bontrager was speaking and I asked him what may have been the cause. He mentioned three things

1. Previous unnoticed crash damage.
2. Poorly sized or adjusted stem causing stress on the bar
3. Something about two different metals coming together and weakening (abrading?) each other over time. It was pretty complex and I can't say I understood it.

Seeing as I used to crash all the time, I think it was number 1 for me.

-s