That is truly a weird phenomenon. I can think of a couple of reasons. First of all, are the radiator/condenser fans(s) still operating at full speed when the A/C is engaged?

Aligned with what jick has already mentioned, water may be clinging on to the condenser fins and restricting the airflow. It's possible that the fan is not or no longer powerful enough (ie. defective or dying fan motor) to pull those liquid out of the airways fast enough.

With the A/C turned on, try to observe the airflow from behind the radiator/condenser fan while everything is dry. Once you have an idea of how strong the airflow is, spray water on the condenser fins and observe the airflow again. There shouldn't be too much of a drop in airflow.