Glad you came out of it ok. From an engineering perspective without seeing the parts it looks like the metal was too thin and the casting was of poor quality. It looks like there is a lot of porosity in the metal which makes it weak. It also makes it full of cracks to start with and very brittle. I was under the impression that most fork lowers were made from billets not cheap castings but maybe I am wrong there. Fatigue should not have been an issue here since there should have been more than enough metal to prevent much deflection. Also the metal should have been made of a rather mild alloy which should not fracture like that unless it is full of holes to start with (poor casting). I have seen this type of fracture in really cheap castings or really weak metal like magnesium. Even if it were a really good alloy I think I would have added a little extra meat in there.