The answer to 'why is my air filter short cycling' is that the air filter is likely clogged or dirty, causing the HVAC system to turn on and off rapidly. This is known as short cycling, and it can be caused by a few common issues.
A dirty air filter restricts airflow, which makes the system work harder to circulate air. This can cause the system to overheat and shut off prematurely, only to turn back on shortly after. Replacing the air filter with a clean one can often resolve this issue. However, if the problem persists, it could be due to a more serious underlying problem like a refrigerant leak, faulty thermostat, or issue with the blower motor. In these cases, it's best to call in a professional HVAC technician to diagnose and repair the system.