What is the most probable cause of a pump spinning backwards after being turned off?

When a pump spins backwards after being turned off, it is most likely due to an issue with the wiring configuration. Specifically, if two of the three wires are reversed, it can cause the pump to rotate in the opposite direction when power is restored. This is because most three-phase motors, which pumps often use, rely on the correct phase sequence to determine the direction of rotation. If the sequence is altered by reversing two wires, the motor will run backwards, which can lead to improper operation and potential damage to both the pump and the system.

The other options address issues that could affect pump performance but are not directly responsible for causing reverse rotation. For instance, a non-closing check valve can lead to backflow issues but does not influence the direction of motor rotation. Similarly, a stuck air relief valve could affect pressure within the system but has no relation to the electrical setup governing the motor's rotation. A clogged mechanical seal water line filter can lead to seal overheating or failure but again, does not impact the rotation direction. Thus, the key reason for the correct choice revolves around the electrical connections and how they determine the pump's operational direction.