An electric motor is an inductive load. When the voltage to it is disconnected, the magnetic field collapses back into the windings, creating a large negative voltage pulse. This negative voltage pulse may somehow be getting back to pin 2, the trigger. As you know, the timing cycle is triggered by a negative going voltage. This may be restarting the timing cycle, meaning the motor never turns off.
Try adding a 'free-wheeling diode' to the 555 output (look up the phrase o...

An electric motor is an inductive load. When the voltage to it is disconnected, the magnetic field collapses back into the windings, creating a large negative voltage pulse. This negative voltage pulse may somehow be getting back to pin 2, the trigger. As you know, the timing cycle is triggered by a negative going voltage. This may be restarting the timing cycle, meaning the motor never turns off.
Try adding a 'free-wheeling diode' to the 555 output (look up the phrase on google). This is connected reverse-biased to the 555 output so that it doesn't normally conduct, but shorts the reverse voltage across the motor when it switches off.