The quick answer is friction.
The oven's recirculation fan moves the air around inside the oven. There is friction between the air and the surfaces in the oven. This friction is constantly trying to stop the air from flowing. Just like rubbing your hands together, this friction generates heat. Being the oven chamber is a highly insulated closed chamber, the heat generated from friction will raise the temperature of the oven chamber until the heat losses through the insulated walls and out the exhaust equals the heat added from friction.
Depending on the type of oven, this temperature rise can range from a few degrees to over 100 degrees Celsius.
The amount of heat added to the oven chamber from this friction is roughly equal to the energy that the fan uses to move the air. For example if there is a 1 HP motor turning the fan in the oven and operating at full load amps, that is equivalent to about a 745 watt heater.
Opening the fresh air and exhaust dampers on the oven if equipped will reduce the temperature rise as this will help remove the heat from the oven chamber.