The lack of a rafter tie is causing the outward thrust. Stating the outward movement on the walls was a bending moment issue would technically be incorrect. Shear forces, bending moments, compression and tension forces are present in all structural members in a house to a degree. Building codes keep these forces within tolerable limits if used properly. I would say the root cause in your graphic was the result of inexperience on the builder or stupidity on the person who possibly removed the rafter tie.
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