There are two reasons why Victor Hugo chose to title Book II of Les Misérables "The Fall."
One reason is because it is in this book that we first meet the story's protagonist, Jean Valjean. We especially learn about his history and how he was arrested for stealing a loaf of bread from a bakery to feed his seven starving nephews and nieces. Hugo makes it very clear that he sympathizes with Valjean and sees him as an innocent man who was severely oppressed by the ironic injustice of the law. The law was particularly harsh on Valjean for sentencing him with five years as a galley slave, simply because he owned a gun and was a poacher. As a gun owner, he was seen as a "brigand," a savage robber (Bk. II, Ch. VI). While Hugo makes it very clear that Valjean is not to blame for stealing the bred, that Valjean's oppressive society is to blame, this moment in Valjean's life can be seen as a fall. Society made him transgress. Not only that, the punishment he received was so severe that Hugo describes it as having robbed him of his soul. It was especially severe because, due to a number of attempted prison escapes, his sentence grew from four years to nineteen years long--all for a loaf of bread. During that time, Valjean judged himself to be wrongfully oppressed, and he condemned society and even God as society's creator. As Hugo describes, "Thus during nineteen years of torture and slavery, [his] soul mounted and at the same time fell. Light entered it on one side, and darkness on the other" (Bk. II, Ch. VII). Therefore, for one thing, the title "The Fall" describes Valjean's spiritual falls both before and in prison.
However, after emerging from prison, the Bishop of Digne shows kindness, mercy, and even treats him with dignity, giving Valjean a moment of redemption. Nevertheless, sadly, immediately after the bishop gives Valjean all of his silver, saying that he has bought Valjean's soul and given it to God, Valjean suffers a second fall, which is also being referred to in the title. After leaving the bishop's home, while crossing some fields, he encounters a young boy named Gervais who is playing knuckle-bones with some coins, meaning that he was throwing the coins up in the air and catching them on the back of his hand, like a coin toss. While doing so, Gervais drops a silver forty sous piece. Valjean put his foot on top of the piece and will not give it back. The boy runs off crying. Valjean was barely aware of what he had done; it is almost as if it had been done by instinct. However, it created a moment of awakening within him, and after trying to find Gervais to return the money and failing, he falls to his knees and cries out, "I am a wretch!" (Bk. II, Ch. XIII). Therefore, we see that this moment was both a moment when Valjean had a spiritual fall as well as a moment of spiritual awakening.