Your very lucky, I was on the interstate years ago and almost the same thing happened to me. Only difference was I was on a long bridge and I tried to stop it but I was doing about 70 mph. The trailer passed me up spun around hit the guard rail and just stopped on the shoulder of the road about 100 yards from me. It was a open car trailer and the only thing that happened was the tongue and one fender was destroyed. The car on the trailer had no damage. I think of that every time I tow now. I guess the man upstairs was watching out for me too because at that speed it could have killed some people and went over the guard rail.