iv been absolutely scrutinising the pics, and i honestly think that casing jumps/flatlanding is the only possible scenario that could have caused this. the frame must have flexed like crazy for this to happen! good to know for next time tho. its also nice to know that its not the welds that failed, it was the material itself.
yeah the rest of the frame is fine, im thinking of doing a dobermann style gusset there now. should be more than strong enough then. at least in that area