Thermostat Issues
Your refrigerator has its own personal thermostat sensor. This thermostat takes readings from both the freezer and refrigerator and turns on the cooling system whenever it senses that the temperature has risen. If your thermostat develops electrical problems, it can start giving false readings, which may end up turning your condenser on permanently, running the fan continually in an attempt to adjust a temperature that doesn't really need to be changed.
Air Clearance
Your refrigerator has vents that the condenser fan uses to pull in air from the kitchen to help cool the condenser and collect more heat. If these vents are blocked by dirt or are covered up with other objects, it can cause the fan to work hard to gather enough air. If your fan is blowing for very long periods of time, it may not be drawing in enough air from your vents.
Condenser Dirt
Condenser dirt refers to dirt stuck on the condenser coils themselves. The fan can pull in dust that, over time, will coat the condenser. The more dust and grime on the coils, the less efficiently the condenser can operate. The result is a condenser that must run almost constantly in order to keep the refrigerator cool. The fan will run as much as the condenser for the same reason. Cleaning the condenser can solve this problem.
Fan and Motor
The fan and the fan motor itself may also have developed problems. An electrical fault in the motor can keep it running constantly, turning the fan even when it is not necessary. If this is the problem, you may need to replace the fan system entirely to stop it from running since the problem typically will involve electrical work within the unit.