This is completely normal. Your defrost is tied into your AC (Your AC kicks on whenever defrost is selected). Your AC evaporator removes humidity from the air making the defrost function more effective. Your fan kicks on to draw air through the condenser mounted on the front of your radiator. It runs to help condense the high pressure high temp vapor into a high pressure high temp liquid to feed the metering device in your AC system.
Factory Trained Chrysler/Jeep/Dodge/RAM Technician