Thoughts on why bow broke?

I was working on this black locust stave, 75". I got it strung (first photo) and unstrung it to work the right limb. Restringing it again and it snapped, where is circled in the second photo. I'm trying to understand what went wrong- no frets on the belly, and it seems the back broke, but I wasn't putting any more pressure on it then I did the first time I strung it. The back was chased to a thick heartwood ring. The stave has some knots, but not where it broke.