There is a reinforcement bracket in behind the inner fender panel. I can't say what caused the crack but I have seen a few of them get thin in that area from sediment laying on the reinforcement behind it. I'd drill a couple holes thru the inner fender and one at each end of the crack and throw a small piece of plate in behind it. Plug weld a piece of 18ga metal in and grind it down and you should be good..