When filling out a request for passport renewal or applying for a passport, the application requests your social security number.  More than likely you will see a statement that says your social security number is requested and will be provided to the U.S. Department of Treasury, and is used in connection with debt collection and checked against lists of persons ineligible or potentially ineligible to receive a U.S. passport.  One agency that is frequently involved is the Child Support Enforcement Agency.

If you owe $2,500.00 (two thousand five hundred dollars) or more in child support, you are not eligible to receive a U.S. passport.  In order to get off of the list, you must make a payment for child support and owe less than $2,500.00, prior to filing your application for a U.S. passport. As it may take a few weeks before Passport Services processes an application, you must follow up with your local Child Support Enforcement Agency to make sure the amount is less than $2,500.00 for the entire time that your application could be processed. The Department of State provides additional information here.

Source: Travel.state.gov