Because there is voltage always present in the (hot) wire this results in an induced voltage in the switched wire, which causes a small amount of current to flow through the LED lamp, and the brightness of an LED depends on the amount of current, not voltage that flows through it. If the driver circuitry is designed so that any voltage present is passed to the LED, the extra current will flow through the LED, thus making it glow faintly. You don't see this with incandescent lamps because the small amount of current that is present is not enough to make them glow.
Normally, placing an incandescent bulb in the same circuit is enough to consume this extra voltage and keep the LEDs from glowing when the power to them is off.
We hope this helps, but feel free to reach out to us either at (858) 846-4973 or submit a ticket if you have any other questions or concerns that we can address.