The slow fade is because there is a large electrolytic capacitor in the power supply which retains a small amount of energy when the power supply is unplugged.

The slow fade is because there is a large electrolytic capacitor in the power supply which retains a small amount of energy when the power supply is unplugged. This slowly drains (depending on circuit) and is why it sometimes take up to a minute to see the LED fade. When repairing or modifying power supplies it is important to be aware that capacitors may still be storing enough energy to cause a shock.