OK, I'm glad that there was a number of responses to this topic. The main reason for the post was to bring awareness to possible hidden damage to a frame. I welded mine in four places due to a poor design and less than perfect road conditions.
In PA, a heavy truck, like the one pictured, is required to be inspected twice a year by a state approved inspection station. If the frame had serious rust or there was visual evidence of metal fatigue, then inspections are a waste. I am not a fan of PA inspections because of the many times that my vehicles were worse after the incompetent inspectors fooled with them. My suggestion is to craw under there and look closely for developing problems, or get someone you trust to do it.
I do agree that the likely cause was metal fatigue. The culvert pipe caused a depression in the road surface, but it had multiple poorly done patches that looked like a small child was at work. The rough patches was probably the cause.