Tube preamp (EAR 868) plays music for 10 seconds after being powered off... Why?

If I leave my sources on and power amplifier on after I turn off the pre-amp, music will continue to play for about 10 seconds while gradually fading out. If I turn off the power amp first (Pass XA30.8), the music stops almost instantaneously. 

Why does the EAR 868 do this? Is it common?
Preamp power supply capacitors provide enough power to run the unit for the 10 seconds. The power amp would do something like that but probably cuts output using relays at power down.
Pretty simple answer. Thanks.