I am thinking fungus. There were also traces of something white between the rings in places.--also, it seems like the absolute back may not have been "real" wood. I peeled the bark off right after cutting and it came right off. Could it be the cambium layer --or some other wood-like part of the bark stayed behind? In the picture below you see two elm staves. The one with the wrinkled/textured back is a sister stave of the broken one. On the other, I scraped this stuff off--it is also a different tree--and came to a harder layer. could the presence of this stuff have caused weakness in the absolute outer layer of the back, instigating the tension break? the Crack runs more than one ring, so I am not sure.