Excessive ice build-up can be caused through several reasons :-

1) Door seal not fitting securely and allowing warm air into the fridge.
2) Manual defrost not done frequently enough.(6 monthly - minimum)
3) Automatic defrost system to functioning.(Thermostat or defrost heater failure)
4) Thermostat set to low.
5) Liquids stored in the fridge not covered up.
6) Door not closing properly or left open for excessively long periods.
7) Badly sited with insufficient ventilation.(in a hot conservatory or next to an oven or oven range)