My guess and I repeat "guess", is that the floor is deteriorated.
If I remember right, the body is wider than the frame, and the outside wall is really only supported by the particleboard floor.. If moisture violates the coating they put on the floor, the board gets soft and spongy.. This allows the body to sag, and as I have seen on some.. The door opening will spread, the door doesn't fit and gaps front to back on the lower side.
This would cause sever stress in the upper corner of the body and could produce this crack.
"IF" that is true, the only real fix would be a new floor, which looks like a pretty big job.
One may be able to pull things back in alignment and add some structure underneath of it to keep it from spreading, but I am not sure what that would look like.
I would be interested to know if my diagnosis is anywhere near correct.