The picture from the link @BuckWallace provided looks to me like a material thinning issue during the stamping process. The drip rails in that area have pretty tight radii in the corners, if the factory received material not to spec, this process was designed maybe a little too close to the splitting of the material (or not accounting for material thinning during cab stress tests), or if someone at the factory on the press just thought "hey imma know better than dem stooopid enganeers" and changed the process set points without validating the impacts on the end panel this for sure could happen.
I hope it's not on my new HD! lol