Like much the internet, we're getting flooded with messages from people who're finding themselves repeatedly logged out of Mailbox, Snapchat (we don't judge!), and other apps following the iOS 7 update. Not only can that be tedious, but in some cases the apps demand the entire setup process be repeated, which can be a showstopper. A bug in Apple's new background update API could be the culprit, according to Ellis Hamburger at The Verge:
App developers are more likely to have problems with their apps shortly after the release of a new version of iOS, but in this case, the issue might not lie with their code. It's telling that companies with tens of millions of users are all experiencing the same issue. Sources say that the issue lies in a new iOS 7 API that lets apps fetch data in the background without being open. Mailbox has explicitly stated that "the root cause lies in Apple's new Background Refresh feature." Turning off Background Refresh in Settings has proven to alleviate the log-in issues, but that also means your apps can't check for updates in the background — one of the most important features in iOS 7, since it keeps your apps up to date even when you're not using them.
You can turn off background refresh in Settings > General > Background App Refresh, and on a app-by-app basis. That could help mitigate feature loss for apps that aren't experiencing the issue.
In the meantime, if that really is the cause, lets hope iOS 7.0.2 or iOS 7.1 fixes this and fast! Apple could get some anger over this, developers could lose customers.
Source: The Verge