Bomb scares aren't the same as bombs. Bomb-threat targets seem to be chosen to maximize publicity and evacuation inconvenience. That's my impression, anyway.
More than likely, it was a student. An academic library will contain some very valuable books. Someone was a little short on tuition and decided to make up the difference with the help of Amazon.com. How long would it take a librarian to account for the entire stock of the building? Give it a few months, a 150-year-old book will be missing.