Could be a number of reasons but the most common is either its wired to a dimmer or similar which is leaking some current when its "switched off" or its switched on the netural side and the wiring is acting as a capacitor and letting some current flow.
Could be a number of reasons but the most common is either its wired to a dimmer or similar which is leaking some current when its "switched off" or its switched on the netural side and the wiring is acting as a capacitor and letting some current flow.
EDIT: https://www.youtube.com/watch?v=_bgUy6zA0ts