More than likely it is from hydrogen embrittlement of the metal. High strength parts require controlled baking after plating to remove hydrogen introduced up during the plating process. Also wrong paint stripper for instance can affect metal. Have seen a landing gear truck break while aircraft sitting on the ramp being fueled. A Service Bulletin had required a dye check of a spot on the truck and the wrong stripper was used to remove the paint. As mentioned too, a machining mark made during manufacturing or overhaul not noticed by inspection can also cause a stress concentration and subsequent cracking.