I'd venture that the reason the crack develops is that the diagonal is in compression rather than tension - for normal stresses - the nose of the bus is to the right in the top picture. If the bus hits a bump the "fulcrum" being the front axle, the nose will want to drop. Dragging the forward (right) vertical down, relative to the left will parallelogram the two verticals, reducing the distance between the two corners of the "D" window.
I could see that. I had been thinking of the member itself cycling through tensile and compressive loads for much the same reasons that you site, but what you bring up could cause cycling this plus add in shear stress as well. I guss if the bus could make it from 1973 to this point, I can repair is to make it a little longer!