It could be a couple of things:
Most likely is that the HDMI CEC is switching on the device. If you go into the settings you will see HDMI or HDMI CEC, switch this off. Unfortunately I do not have this model box at home so I can't give the exact instructions to locate it.
An app is causing it to switch back on at certain times. If you have installed any 3rd party apps then try uninstalling them or do a factory reset.
Unlikely but a possibility, it might be your TV remote or another IR signal. One of the buttons IR signal is similar to what the device can respond to while switched off.