Answer

If you have a mobile device connected to the 3.5mm Input and turned down, there may still be enough crosstalk that the loudspeaker is detecting light signal. Likewise, if you have Input B set to HI-Z for a passive instrument, and have unplugged that instrument, there will be enough noise present on that High Impedance input that the loudspeaker is still detecting signal (even though that signal is noise). In any case where audio signal (including noise) is detected, the amplifier will not go into Standby.