Thinking about what might have caused the crack, given that the brake calliper is cast out of the same magnesium alloy would you not expect that to crack before the more substantial mount? I'm no engineer, but there's a lot less material/strength in the calliper. Would this not point to a flaw in the fork casting?
I would tend to agree - when the sliders are cast there will be a cored hole cast into them, which is later drilled to size/tapped for the caliper mounting. It's done this way as it's stronger than casting them solid and drilling out completely. As the molten metal has flowed around these cores it has failed to properly meld with itself at the flow front and so you get what is called a "weld line", a weak point which has afterwards started to open up.
Usually caused by the mould or cores being at too low a temperature, injection being too slow, air entrapment or insufficient back pressure.