If the same as a car wiper, there is a home position switch. In this position the power must be applied by the windscreen wiper operation switch/circuitry to move the motor away from the home position. Once it is away, a constant supply voltage runs the motor to allow it to sweep return to the home position.
Thus when the wiper is away from home position, turning it off will not stop it operating until it returns to engage the home position switch.
If the home position switch is faulty, the wiper could be able to run by itself continuously.