Well there you go! You can see the tiny little spot welds from the factory. Doesn't look like a lot of rust either, so that's good. There's a crack at the inner fender sheet metal there, so probably what happened is the upper weld didn't penetrate all the way through to the frame rail so the upper part of the plate was only attached to the inner fender sheet metal. The inner fender flexed until it cracked, and now the upper part is loose.
You should be able to just clean that up with a wire wheel, then add some welds to re-attach/reinforce the plate. It won't take much, you could either put some welds on the outside lip of the plate (super easy), or if you wanted it to look more original you could drill a couple holes through the plate to the frame and plug weld it back to the frame. In either case, it's not a repair that should take a lot of time. Or skill either, you just need someone with a welder and a minimum level of competency using it.