If the air flow is too slow through the central air unit, then it does not transfer enough heat to the fins of the exchanger (cooling the air and heating the refrigerant). Without enough air, the exchanger gets colder and colder, and ice tends to build up on the fins. This problem is often caused by debris blocking the air way, or problems with the fans that circulate the air.
The refrigerant is a gas inside the air conditioner that helps transfer heat back and forth. If the refrigerant is low, which can be caused by a small line leak, then it will move through the system very quickly, more quickly than the air conditioner was designed to handle. As a result, it cools down the unit too fast and ice builds up.
Sometimes refrigerant levels themselves can be fine, but the mechanisms that sense refrigerant pressure or temperature can fail, letting the refrigerant pass onto the next part of the air conditioning unit too quickly. When this occurs, the unit experiences the same sort of problems that it does when there is too little refrigerant, and problems occur.
Defrost cycles reverse the movement of an air-conditioning system in winter, briefly bringing it cool air so that it can cycle warmth throughout the unit. It does this periodically so that frost will not develop in the unit when temperatures fall below freezing. If the timer cycle is off or the defrost system is not functioning correctly, then ice will develop due to naturally freezing temperatures.