The iPad can do several things in the background when on a wi-fi connection, like sync your Photo Stream. If it's having authentication issues it will probably popup with a password request. This is often a temporary server issue.
If it is consistently bad over several days:
My recommendation is to go into Settings > iCloud and sign out. Wait a bit, then sign back in.
When asked if you want to delete your contacts, events, etc on the iPad say yes; otherwise you may get duplicates when you log back in and it syncs. It's not a bad idea to visit icloud.com on a computer first to make sure everything in iCloud looks good before doing this.
You may have to turn back on Use iCloud in your iWorks app's settings afterwards.