I'm guessing banked corner, going real fast, leaning very hard into the corner and powering the pedals must have put a lot of twisting force through the frame causing this, the corrosion didn't help though.
That would have been the final straw, but it's probably been cracked for months, if not years - it'll have started on that bit that's right on the weld for the chainstay bridge and gradually got longer. That can go on for ages until eventually there's not enough metal joined together to take the loads and the rest of it all goes at once.