It looks like there's another bug in iOS 8. This one was found by MacRumors forum members and causes you to lose all your documents in iCloud Drive.
The bug creeps up when you select Settings > General > Reset > Reset All Settings. Typically, this is just supposed to reset your network settings to give your iOS device a clean slate to work with, but it turns out it's also deleting all your files from iCloud Drive. It really seems like iCloud Drive just isn't ready for prime time yet (and that Apple is being reminded how hard it is to keep software bug-free). If you've already enabled it, make sure you don't tap that "Reset All Settings" option.