Dave,
That is pretty wierd. Must be some strange metallurgy going on. I have brazed a bit of cast iron and I haven't seen it crack upon cooling. The bronze is pretty soft and ductile, so I would think that it would stretch first before the cast cracked. The old welders trick is to peen $hit out of the bead with the pointy end of a chipping hammer while the bead is cooling. This mechanically expands the metal so that stress doesn't build up.
No doubt in this case it's operator error. ;p