Brakedwell is the man who can explain exactly what happened.
Sod's law at work I 'm afraid. A newly converted Captain with a new Flight Engineer. The F/E did not fully engage the reverse arm lever in it's detent, so forward thrust was active when he moved the throttles back. I understand several attempts were made to achieve reverse, each time compounding the problem with more forward thrust until they went off the end of the runway.
