I see this behavior on my dual-DP setup, where I leave the (work) computer running overnight, but turn off the screens to preserve some power. And also resets the other virtual screens to something like 640x480 pixels.
Well, with modern display-drivers and screens, it seems the entire display is inhibited when in standby/off, and Windows shakes/resizes all it's visible windows to the last, or the primary, awake monitor it still sees.
