I don't think a spring breaking can be blamed on rust. There's no place to trap water, especially on that loop. I would guess it's metal fatigue from overloading, or maybe a manufacturing defect that took a while to show up.
I am guessing overloading was the cause. This was a farm truck. The airbox had field corn in it when I bought it.
Most farms that I have been around - if you can put it in the truck or hook the hitch up to it, it isn't too heavy...
I do still hate rust though. The good news is that after I removed about 100 pounds of dirt from the underside of the truck, the rust isn't too bad. Being off the main roads and in farm duty has it's pros and cons.