One thing I do know and can see is that it is under designed and had a poor tiller judging by the pic you posted....how wide and long were the working portions only? Regardless of what it is tho your bend needs to be perfect with lever bows otherwise bang!!! And its even more so with Whitewood's....and regardless if it was rot or not try making your limbs wider and distribute the bend better before pulling it that far....if you were pulling it that far like it is in the pic then I am not surprised it broke,and that's even if it was good wood(no rot).