A window air conditioner freezes up for a variety of reasons. The most common reason is a temperature that the unit can't deal with. However, there are several other possible causes. One common cause is a dirty filter. When the filter is clogged with dirt and dust, it can prevent air from flowing through the unit properly.
Another common cause is a blockage in the condenser coil. If the coil becomes clogged with debris or lint, it will prevent the unit from cooling properly and can cause it to freeze up. Finally, if the refrigerant levels become too low, it can also cause the AC to freeze up.