Header failures are not rare in the 100+-year-old masonry construction of Capitol Hill.
In some cases, the reason for failure is as simple as natural deterioration with age.
If you look closely at the picture, you can see there is a vertical zig-zagging crack in the brick and brick mortar, where the structural load path has a differential settlement.
If you look closely at the eyebrow or segmental arches above the windows in the picture, you can see that the bricks in the soldier position around the segmental arch have shifted over the years and are falling out of place at specific locations.
In other cases, contractors who did not know about working with historic materials have used the wrong materials and it actually accelerated the degradation of parts of the building assemblies. We covered spalling in another post.