The answer to 'why is my furnace short cycling' is that your furnace is turning on and off rapidly, rather than running for a normal cycle. This can be caused by a variety of issues, including a malfunctioning thermostat, a problem with the furnace's air flow, or an issue with the furnace's internal components.
Some common symptoms of a furnace short cycling include the furnace turning on and off frequently, the house not reaching the desired temperature, and higher energy bills. If left unaddressed, short cycling can cause additional wear and tear on the furnace, leading to more serious problems down the line.