From the look of the way the section of flange just under the crack/break is pulled away from the inside of the frame rail I'd be willing to bet it never fit properly when it was originally put together. Then after a few years of twisting and working a crack appeared due to the added tension of the poor fit.
Just my $.02 for what its worth. I suspect if its welded back together just the way it sits there will be no more odd stress on the joint and there's a good chance it won't crack again.
