I've had the same problem. It didn't do that previously, there used to be a back button at the top on the left hand side, but it's disappeared. There is a quicker way than shutting down the whole iPad, if you have a relatively "new" one (not iPad 1) - double click the home button to give you a scrolling sideways screen which shows snapshots of all the apps you have open - if you flick the relevant app upwards so that it disappears off the screen, that shuts that particular app off, you can then single click the home button to get back to the normal screen, and re-start the app.