The answer to 'why is my dehumidifier short cycling' is that the dehumidifier is turning on and off too frequently. This is usually caused by the dehumidifier being too large for the space, a dirty air filter, or the humidity setting being too low.
Short cycling can lead to higher energy bills and potential damage to the compressor over time. To fix it, try adjusting the humidity setting to a higher level, cleaning the air filter, and ensuring the dehumidifier is the right size for the room. If the problem persists, you may need to call an HVAC technician to diagnose and repair the issue.