Sorry. Probably should have been more specific, when you touch the cast button, you get a number of options of devices currently playing music (or available to be airplayed to), touching a playing device gives you the ability to remote control the playback. If you touch on the extreme right of said device, you get an expanded set of options as below.
Touching the end option with the 2 downward facing arrows transfers the playback to the current device. Including the forthcoming tracks in the queue. Does that clear it up for you? Hope so. I know Spotify has somewhat similar behavior when all controls across devices are synced whether you want it or not.
