Silent mode but iTunes Music is still playing

I attended a concert last night and as usual, I silent my iP5s. After a while, my partner informed that my phone is ringing. To my surprise, my iTune music is playing despite the silent mode is on. I did not select iTunes Music throughout and the phone is in my pocket. I took out my phone and close the app and to be on the safe side, I turned down the volume to zero. How can I ensure this don't happen again?