That is how fatigue works. A crack initiates at a stress concentration on the surface, caused by a gouge, or sharp transition, or corrosion pit, or even weld, etc., due to cyclic loads far below the design or ultimate loads or stresses the part is designed to handle. Once the crack initiates it continues to grow as the loads continue to cycle, creating beach lines emanating from the initiation site outward until finally there is not enough intact cross section of material left to carry the ultimate load and the part fractures due to overload with fast fracture. Parts are usually designed to have a fatigue life of generally 2 times the design life of the part to ensure that the part never fails from fatigue, but a design error or manufacturing flaw can cause the failure within the design life of the part. A skid plate designed and manufactured correctly should never fail from fatigue in your lifetime on a ground vehicle. The fracture surface in the photos is unmistakable showing fatigue cracking until finally fast fracture of the remaining material. The primary cyclic loads in the case of the skid plate, if it is only attached to the engine and not tied to the frame, are thermal cycles due to the temperature cycles every time the vehicle is warned up and then cooled down due to the thermal expansion and contraction of the engine, and likely also to a lessor degree the inherent vibration of the engine.
