Nephi had set up this story by indicating the importance of the bows to the provision of food for the family. Now he provides the crisis. His bow broke. From a literary point of view, it is interesting that he doesn’t mention what happened to his brothers’ bows until a few verses later. It is probable that as Nephi wrote he knew that this was the incident he wanted to tell, and it is a story about Nephi. Therefore, he begins that story.
However, Nephi realizes as he had written that a reader wouldn’t understand why the family had no food if only his own bow had broken. Why not simply use his brothers’ bows? He has to explain, therefore, that those bows had already become unusable because they had lost their springs, meaning that they no longer had the sufficient pull strength to launch an effective arrow.
What Nephi doesn’t explain is how his steel bow broke, or how the other bows lost their springs. First, the steel bow might have had metal as part of its construction, but bows at that time were not completely made of steel. There was wood involved. In the change of climate, the wood of all the bows dried out. The brothers’ apparently completely wooden bows lost their power, and Nephi’s bow probably warped, and broke as he pulled it. This would not be unusual given the timeframe and environment, that possibly had much less moisture in the air.