Why did Lehi murmur against God?

Because there was no food to eat after Nephi broke his steel bow.

And it came to pass that as I, Nephi, went forth to slay food, behold, I did break my bow, which was made of fine steel; and after I did break my bow, behold, my brethren were angry with me because of the loss of my bow, for we did obtain no food ... and also my father began to murmur against the Lord his God. 1 Nephi 16:18-20