Sorry to hear that John. It always sucks to break one.
Looks like the backing was the start of the problem. You say it broke at the same spot on both sides simultaneously? That's interesting...
One good thing about bamboo is you don't have to worry grain. You have other things to worry about, but not grain runoff.
And every time I've broken ipe, it cracks with a puff of dust, and then it looks dry alone the fibers. Inside my garage was 84% humidity today, and I used to not measure moisture content, so I know the ipe wasn't too dry.
What taper rate did you use?