Are you sure your fan didn't fail, they have a nasty history of crapping the bed and taking out the rad with them. They are known to sieze and spit out a bearing into the rad, leaving no visable damage on the fan, but it will feel siezed if you try to free spin it.

For pressure to build up and blow a hole in the rad you will need a pressure source, and for pressure that high a blown head gasket is a likely suspect, and faulty rad cap that prevented the pressure release. Basically there has to be something majorly wrong for enough pressure to build up to blow a hole in a rad core!