To clarify, have you tried exiting the app when you are done streaming Pandora?
To quit the app without signing out of your account, bring up your list of currently playing apps (either by swiping up on your home screen, or by pressing your Home button twice – depending on what kind of iOS device you have).
Swipe the screen to the left or right until you find Pandora. Once you see Pandora, swipe the preview up and it will disappear. The app is now completely closed.