The only clue I see is the deformation at the top of the crack (A). Something pushed that out from the inside.
But I agree that doesn't explain the rest of the crack. It doesn't make sense, given the absence of obvious stress in the plane of the sheet at that location. If the steel was brittle, a crack could propagate but it's not. Like a bullet hole, the puncture itself won't cause that crack to form.
The "stepped" crack edge at B is also unexpected... not typical for ductile low-carbon steel. Almost looks like stress corrosion cracking, but again, that makes no sense here.
It does appear the side of the crack following the line "C" is folded inward... is that correct?
This one will likely remain a mystery. Perhaps it was a manufacturing defect related to stamping the opening for the extractor? Or maybe it got ripped by something and was previously flattened by someone, "destroying the evidence"?? The rusted grinding marks tell you someone worked on it.
Because you can't explain it by "overstress" in that area, I personally don't think you should worry about the repair failing, but definitely weld it, even if it needs a patch.
Good luck with it!
