Could just have been internal stresses. The one I made the aforementioned plug repair for, was a bike that was equipped with a coaster brake. The crack was in the seatstay, which I surmised had some internal stress put on it by the coaster brake's reaction arm pulling on the chainstay - putting various loads on the dropout and the seatstay. The same sorts of stresses could potentially occur if the dropout faces weren't parallel but were squeezed together to be so, or if the frame was ridden with a broken axle held together with just a quick release, or even a bent axle, etc.