That appeared a few times in the past week(s) already. Might be due to the last software update.
I noticed that one (randomly, not always the same) of my 4 soundtouch 10 would wake up by itself - usually a dozen of minutes after I turned it/them off. It seems the one who wakes up is the master of the group/ the one I turned off just before - but not 100% sure.
Not sure either if this is a systematic bug, as usually, I turn them off in the morning and then leave the flat, thus I don't know what is happening while I'm out.