iOS 7 automatically downloaded to phone after downgrading to iOS 6
Your iPhone software update is trying to be smart by downloading the update in the background for iOS 7. What it is trying to do is make sure that the update is already on your device, and all you need to do is press the update button, without waiting for a download to finish.

You aren't in danger of loosing your downgrade - your phone will always ask your permission before installing any update. You probably won't be able to keep it from downloading the update, though.