Due to the steel plates not being kept in ideal conditions, this effect can happen when storage has been vertical, (say up against a wall), and rainwater has run down them and created initial rusting, and the plates have been welded on horizontally. Also, if the plate was at the bottom of a pile and other plates have been dragged across it when removed. Remember that rust proofing was minimal, a quick wire brush and a coat of wagon colour, job done.
Mike.