This means that the Pandora is quitting/crashing/closing as evidenced by the fact that when you wake the iPad it has returned to the home screen. If the app merely stops, when you wake the iPad, the app should still be open. At least that's what I think is going on.
I didn't even know what Pandora One was until I did a little quick research on this site. I see nothing about any time limit other than if you have free Pandora like I do. If you paid the $36.00 annual fee, threshold be no timer involved - if I read correctly.
I entered "is there a time limit" in the search area to find the information on this FAQ site.
http://blog.pandora.com/faq/
If in fact the app is closing, I would try deleting it and reinstalling it. The app is free, the subscription service is what you pay for unlimited hours - correct?