Cracks go diagonal when a wall is being twisted, not stretched.

Diagonal Cracks: diagnosed and explained.

Crack direction is one of the most useful clues a house offers, and diagonal is the direction that points at the foundation. A wall that shrinks, from curing lumber or seasonal humidity, cracks vertically along seams. A wall that is being pushed sideways cracks horizontally. But a wall where one end is dropping relative to the other is in shear, and shear failures run at an angle, typically around forty five degrees, from the points where stress concentrates. That is why diagonal cracks so often start at the corners of doors and windows: the openings interrupt the wall, so the racking stress funnels into their corners and tears outward from there. You will see the same geometry indoors in drywall, outdoors in stucco, and in brick as stair stepping that follows the mortar joints diagonally. The force creating the shear is differential settlement, one section of foundation sinking while its neighbor holds, and in Southern California, Arizona, and Nevada that is usually expansive clay shrinking and swelling with the seasons, erosion from poor drainage, or fill soil still compressing on a hillside lot. A short, stable hairline at one window corner may be original stress relief. Diagonal cracks that lengthen, widen, multiply, or pair up with sticking doors and sloping floors describe active movement worth measuring.

For the repair side of this, the foundation repair solutions page lists every method Dalinghaus installs, including the ones that fix problems other than diagonal cracks.