I've only noticed this happening when I'm using the iPad app and it definitely only starting happening over the last month or so...
I’m in the lounge which is a pair of Play1s and was listening to a radio station and was controlling it with the sonos app on my iPad. The Sonos app is left full screen and the iPad allowed to go into sleep mode. The same radio station is still playing in the lounge with no problems.
When I then power the iPad back up again the Sonos app is now showing Master Bedroom and a different radio station and the Play button is visible because that Zone isn't playing. If you don't notice this and press the play button (thinking you're pressing stop) then it starts of the Master Bedroom zone.
It's always the Master Bedroom that it swaps to when it goes wrong like this. The Master Bedroom is the next in the list of zones but I hardly ever use that zone.