It's hard to say what caused it. I planned on the tear weighing 1500 pounds... it ended up on certified scales at 1960 with the mattress in it. That's total weight. Add the camping gear, subtract the tongue weight, and I guessed it was close to the 2000 pound spring rating. I don't know how much if any margin was on the 2000 pound rating on the original springs.
We have over 4500 miles on it, with a lot of dirt/ rocky road miles.
The broken edge was right at the center bolt hole. Maybe there was a crack started from the drilling? Maybe not.
That is also the side that we lost the tire on a year ago. Not sure if many miles of running on a rim would have contributed to cracking the spring.

Anyways, with a broken spring, I went up a size to the 3000 pound ones.
Sam