Yikes. I worked with a guy whom I later figured out had narcissistic personality disorder. He was very intelligent and charming, but was prone to excessive talking, especially when nervous. A lot of that had to do with the fact that he was a sociopath who had very little interest in other people -- only himself. I can now spot someone with NPD from a mile away. I avoid them like the plague. You should too. And it looks like you certainly did in this case, thankfully. Here's what the shrinks have to say about excessive talking:

Several mental illnesses frequently involve compulsive talking, including:

Bipolar Disorder: As a mood disorder, Bipolar Disorder can present with compulsive talking. This symptom typically occurs during manic episodes rather than depressive periods. It is often characterized by rapid, incessant speech that leaves little room for thought or even breathing.

Narcissistic Personality Disorder: Compulsive talking may be a symptom of Narcissistic Personality Disorder. In this case, compulsive talking often manifests as a relentless need to self-promote or belittle others. For example, an individual with this disorder might compulsively discuss their accomplishments and achievements, regardless of their audience's comfort or interest.

Schizotypal Personality Disorder: Compulsive talking can also be a symptom of Schizotypal Personality Disorder. Individuals with this disorder may compulsively discuss specific ideas or beliefs, such as conspiracy theories, ESP, or paranoid thoughts. Their speech may be peculiar or unusual, characterized by odd phrasing, disorganized speech, or unconventional terminology.