Its clear it went over the edge with very little forward speed, or it would be a lot further down and more damaged.
My money is on - They were doing the backtrack turn (as post 70) and put the nosewheel on the grass, at that point they would have had no nosewheel steering to speak of. Then symmetric thrust just carried them straight on over the edge at taxi speed. Possibly combined with TR being taken out with remaining N1...