Two common reasons:

1) Freon levels are low or leaking! 
Freon is what collects the heat within your home and moves it outside. When Freon levels are low or leaking, the coils get too cold and then freeze as the humid air moves through the system.

2) Not enough air flow!
Not enough air flow causes the Freon to build up in the coils. When humidity hits the coils, the low temperature mixed with the moisture, causes the coils to freeze. This is usually caused by dirty filters or coils and clogged or damaged duct-work.