The backup was encrypted because you checked “Encrypt Backup” in iTunes at some time in the past. If you have a company MS Exchange email account on the phone your Exchange admin may have checked it for you, but in either case the first time you made a backup after it was checked you were prompted for a backup password — TWICE. That could have been years ago. That password that you entered never changes, and you are never prompted for it again until you want to restore the backup.
The good news is that you get unlimited guesses, so think about what password you might have used back then. To speed up guessing click Change Password.