this same thing happened to me on a touring trip from brooklyn to philly. all of a sudden my miyata 610 rear wheel would pull forward almost out of the rear dropouts. happened a few times. scary. the on the fly fix i found was odd. i originally had the QR skewer handle pointed rearward to stay out of the way of panniers and straps etc. i tried it facing forward and success. absolutely makes no sense to me. i haven't investigated why this worked but its stayed in that position for over a year with several longer touring trip and no problems.