Likely the unit is set to the Auto On Mode switch position. There are 3 ON Mode Switch positions that are vital depending on the functionality of your unit. Trig/Auto/Ext Cmd. This can be adjusted via the silver toggle switch at the back of the unit near the power connection.
Trig: Sets the MDX to turn on and off when it receives a voltage signal from another connected device via the “Trig In” connection. The MDX will turn on when a signal is applied to “Trig In”, even if no audio signal is detected. The unit will enter standby mode after the Auto-Off delay once the trig signal is removed and no audio signal is detected.
Auto: Sets the MDX to turn on when it detects an incoming signal from a source device on any of its digital or analog audio inputs.
When the MDX stops detecting all incoming audio signals, it enters standby mode after approximately 20 minutes (by default). Off and on times can be adjusted via the web interface.
Go to General > Preferences > Auto-Off Delay
and Auto-On delay.
Ext Cmd: Sets the MDX to ONLY turn on and off when it receives commands from a control system via an IP or RS-232 connection. This is the required position when using third-party IP or RS-232 control systems.