The housing market is filled with risk and volatility, which Fannie Mae's employees know well. A solid understanding of the major financial crashes of the 20th and 21st centuries is expected of you. You should be well-versed in the mortgage crisis of 2007 and the housing market crash of 2008, and how these crucial events impacted Fannie Mae's subsequent government bailout and conservatorship.
On a micro level, analysts must always factor potential risks into the equation for any project they work on. To get a better idea of how you will do this if hired at Fannie Mae, your interview is giving you a chance to talk about your knowledge of risk and how you have worked with risk mitigation and risk avoidance in your previous positions. If you're new to the industry, it's important to show that you know that common risks can be systematic, operational, legal, and credit-related.