The Move (off the charger) appears to have two power-down modes ‘standby’ and ‘deep sleep’ the latter of which is almost like a powered-off state, although it may occasionally wake itself up for updates and IP address renewals etc. To save battery, whilst off it’s charger, the Move falls into ‘standby’ mode after 30 minutes of inactivity.
In general/normal ‘standby’ mode the speaker will wake when the Sonos App is opened, or it’s placed on it’s charger - that feature is ‘by design’.
If you don’t want that to happen, you may need to put the Move into deep/sleep (power-off) mode. It needs to be off it’s charger-loop to do that - Just long-press (5+ seconds) its top rear power button until you hear the deep-sleep/power-down tone. Another option is just to simply leave it on its charger and it then does not go into standby at all.