I have no idea what triggers this, but if I'm spectating a game and then disconnect from it, every once in a while I'll still get a feed from that game.
I'll go back to my own game, cleared board and then a card will suddenly show up on the screen as somebody plays it and I'll get messages in the chat from the game I'm no longer watching.
When it happens I have to disconnect from the server completely and reconnect in order to get the other game to finally release.
This has happened to me too.