Fact or Fiction--Trailer tie downs

was traveling through Maryland south and was approaching the bridge that goes over the Chesapeake. My buddy was driving this stint....i woke up and looked in the rear view and noticed the front end of my duster was moving up and down WAY to much as we drove up the bridge and over the expansion joints....
i told my buddy ...something is not right...pull over..he said...ON THE BRIDGE? i said..
take it easy then pull over ASAP
the front pumper had sawed through the front tie strap..the only thing keeping the car on the trailer was the park prong..
was using a borrowed trailer so my tie down routine and chain back up where not on hand...
i keep my chain back up with my race gear...now...