It looks like there was an inclusion in the wood right there running across the limb. Just to the right of the fracture, right up against it practically, there appears to be the remnants of some type of pin knot, change in grain, or something similar running across the width of the limb.
That's the risk you run with quartersawn wood. Any little flaws like that will run the width of the limb and are ultimately 'more fatal'.
Utilizing such a piece with a totally flat belly/rectangular cross section seals the deal.
My guess is, this inclusion would have been much less a factor, you'd have likely never known it was there, if the piece was rift sawn and the bow's belly was fully radiused.