The answer to 'why is my furnace freezing up' is that a frozen furnace is often caused by a lack of airflow through the system. This can happen when the air filter is clogged, the blower fan is not working properly, or there is a problem with the refrigerant levels. A frozen furnace can also be a sign of a more serious issue, like a malfunctioning compressor or a refrigerant leak.
If your furnace is freezing up, the first thing you should do is check the air filter and replace it if it's dirty. You should also make sure the blower fan is running and that there's no ice buildup on the coils or other components. If these basic troubleshooting steps don't solve the problem, it's best to call in a professional HVAC technician to diagnose and repair the issue.