Why does your car over heat when you turn on air conditioner?

The air compressor engaging adds resistance to the turning of the engine crankshaft. This means more effort is required of the engine in order to compensate for the added resistance. If this causes your engine to overheat, it could be for a number of reasons. Weak water pump, insufficient coolant in the system, ineffective thermostat, blockage in the cooling system, insufficient airflow through the radiator cooling fins, or it could even come down to your car being a POS. It's hard to say without a proper diagnosis.