I had a similar event during my training process. I was also taxing back to the pit when all of a sudden it went full throttle into the fence. I got luck that it was about 2-3 feet from the fence so it did not build up a lot of speed. Hit the edge with my wing and just put a small dent in it. The cause: As we were moving to follow the plane during the taxing I stepped on the cord a pulled it off of the buddy box. We were very careful after that.