Using clues from the novel and my handy brain, I figured out that Javert had Obsessive-Compulsive Disorder. Here's my explanation:

THE OBSESSIVE PART: Obsessions are what someone is fanatical about, (a) certain thing(s), like cleanliness or perfection. Javert was obsessed with being the perfect epitome of Justice.
THE COMPULSIVE PART: Compulsions are actions done to satisfy obsessions. Like, frequent hand-washing to ensure cleanliness. In Javert's case, his compulsion was the continual chase after Valjean to ensure that he remained the perfect epitome of Justice.

When obsessive-compulsives are unable to perform their compulsions to satisfy their obsessions, they often become aggressive, or uncontrollable, and sometimes they may even be driven over the edge into insanity, depending upon the severity of the individual's disorder. (Thus the reason for medical treatment of OCD -- obsessive-compulsives can often be harmful to people around them, if untreated.)

So what exactly happened with Javert? Well, because of the incident at the barricades of the student insurrection, he was unable to remain the perfect epitome of Justice. This drove the inspector over the edge into insanity and consequently, over the edge of the bridge into the [16-inch deep] River Seine.

So that's my theory. Did anyone seem surprised to read this?