[bstark]

I may be in error but my take on the explanation is NOT the welding itself but the placement and type of suspension that has offset all of the trailers weight from under the frame members to their outsides. which has served to cant them apart at the bottoms. I can see how this would occur if no attempt were made to 'tie' the two frame members parallel at the points where the suspension is providing this 'lever moment' to the frame.

I dislike any suspension system that displaces the weight and support of the trailer to an offset positon equal to the distance of the wheel from the frame member instead of directly 'under' those frame rails where it belongs.