One thing that a lot of people don't take into account is a rubber tired load will bounce a certain amount as you go over rough spots. When that happens and the load comes up again after the tires have squatted, the load imparted to the hold downs will be greater than what it was in a static situation. If that bounce is great enough it is possible it can cause the hold downs to break, even fairy hefty ones, and don't forget...the angle of the hold downs between trailer and resultant loads also influences the load on them and can magnify the forces. Vectors and geometry and all that stuff.