Most likely it was a bit of differential temper along the blade, with the critically weak transition being where the break occured. Again, it is the way the latticing forms in the cast, then how the tempering process is applied that accentuates that temper line just enough to where something like this will happen in a highly structured alloy like D2. Softer steels have to really be temper processed to form such a break line, but it can be done. I've had chisel tips and punches snap at the temper line like that as well.

Back in shop, we were always told to keep the tool moving around in the liquid when we were tempering it so that you didn't create such a breakline, the idea being that the liquid would move up and down along the hot metal and this would shed the temper differential across a greater area. I'm cetainly no metallurgist, but it seems sensible to conclude that was at least a contributing factor.

Or maybe the dang thing just broke...