Every time I try to download an update (iOS 9.2.1 in this case) on my iPhone 6 that I bought 4 months ago, it says I can't because my phone is still restoring from an iCloud backup. Why is it still restoring? The little wheel at the top is constantly going around.
Settings > iCloud > iCloud Drive and turn that off - This worked for me after turning it off, back on, then off again. Restore stopped and then i turned it back on and did a back up.
Likely your phone does not have the storage to completely restore your iCloud data, photos etc etc. Or the 5GB you get for iCloud isn't enough to backup your phone with the default settings.
Go to Settings > iCloud > Photos and untick Download and Keep Originals
Go back to Settings > iCloud > iCloud Drive and turn that off too. If it continues to 'sync', then try progressively switching off synced items in the 'Settings > iCloud' menu with 5 minute pauses in between, and let us know which is the culprit. You can then go to your browser on a computer and download or remove that synced data. Truth is it could be anything from non-standard letters in Safari bookmarks, to maxed storage on either iCloud or your phone, to iCal dates that accidentally set themselves for 4725AD.