There was as trailer truck that had delivered some gardening supplies to my house. I had some left over and so they were returning them to the garden supply store. I guess I had followed him in my car. He pulled up along the front of the store but he stayed in the street. Along one side of the store was a chain link fence. On the outside of the truck was a metal frame. The trailer had a metal frame with canvas sides that you could unhook to get to the stuff that was on the trailer. I had climbed up on the outside of this trailer to unhook the canvass so we could unload the extra stuff that we were returning. I was trying to hold on. There were two places, one at the top and one at the bottom where the canvass needed to be untied. Then there were two places in the middle that had metal clips that you had to squeeze to take off of whatever it was attached to. I was concerned that he had left the truck in the street because I knew it was blocking traffic. I wanted to get it unloaded as quickly as possible. I was trying to hold on with one hand and undo the canvas with the other. I had let go with both hands because I needed both hands to undo the canvas, then I grabbed back on. After I did it I thought it was strange that I didn't fall when I let go with my other hand. I got the one side undone and the truck started rolling forward. I got in front of it and just pushed it with my hands back some so it wouldn't roll forward. As I did this the trailer moved at an angle towards the fence. I actually think it bumped up against the fence. That also stopped it so it roll again. There were 2-3 men that came out of the garden supply store and picked up all of the stuff that I was returning. There were also some little things like little metal clips or something that the also picked up. I think I picked up one of those and the last bag of whatever it was that I was returning. They were all bags of peat moss or mulch I'm not sure. As they grabbed everything and got it off the trailer and were returning it I made the comment that there was one bag of something else that I needed to take with me.