Just in case anyone is interested in the info, I called CIC and asked what about my application triggered this request. The letter references requirements of sections 21 and 22 of the Citizenship Act. Section 21 states that time spent incarcerated or on probation does not count towards residency requirements. Since my only brush with the law since I've lived in Canada (about 15 years now) has been one minor speeding ticket, I was concerned that there was a mistake somewhere that flagged me as possibly having been incarcerated. Section 22 broadly states certain conditions could prohibit one to become a citizen. Since it's requiring a fingerprint search, I'm guessing that is referring to certain crimes being committed.
I was told by the agent that there wasn't anything specific, but rather just that random applications are selected for fingerprint verification. So, that'll be $70 and at least a week long delay in getting the results back. Fortunately I only need to send them back to the Toronto office.