I used to work in a Freightliner dealership and had a cabover come in with a very simular problem. Both frame rails were nearly severed, with only an inch or so of metal holding them together at the top. It looked like stress cracks started at the bolt holes for a crossmember and just continued to grow.
I would blame over loading, rough roads, and old stressed steel. Of course a good pre-trip insection should have caught this sooner.
The shop forman said that frame rails on a truck move just like bending a wire back and forth to get it to break. After so many times being flexed too far it is going to break!