The answer to 'why is my evaporator coil blowing cold air' is that the evaporator coil is likely frozen. When the evaporator coil becomes too cold, it can freeze up and start blowing cold air instead of cool air. This is a common issue that can be caused by low refrigerant levels, a malfunctioning blower fan, or a clogged air filter.
If the evaporator coil is frozen, you'll want to turn off the HVAC system and allow the coil to thaw completely before restarting it. Once thawed, you can try replacing the air filter and checking the refrigerant levels. If the issue persists, you may need to call in an HVAC professional to diagnose and repair the problem.