Driving DC motor using a single MOSFET, why does the motor spin without applying a gate voltage?
By "no gate voltage applied", I assume you're not driving it - it's unconnected, or connected to high impedance such as an input pin.

A MOSFET's gate has extremely high impedance - from hundreds of megaohms to gigaohms - and thus it takes very little current to change the voltage on it. Ambient EM fields can easily affect it, and with nothing connected the voltage can fluctuate and take on just about any value. In your case it's likely above the Vgs threshold voltage, turning the MOSFET on.

This is why you should always have a pullup or pulldown resistor on your MOSFET's gate if there's any chance it might be otherwise undriven.