Back UP

Take a look under Edit > Preferences > Devices. You should see something a bit like this:

The backups with the padlock symbol are encrypted. If a device is already set to make encrypted backups and you connect it to a new computer to back it up, it will make an encrypted backup using the existing key without any prompting.

When you want to switch between encrypted or unencrypted backups you use a check box on the summary tab like this one:

Ticking that box when it is unchecked will prompt for a password. If already checked the right password has to be supplied when unchecking it or using the change password button.

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I can only assume that sometimes people enable this option without being aware of it and supply what they think is a required password at the time. They won't be asked for the password again until they want to restore, so it is easily overlooked. When you look at the backups available to you do they show with a padlock that indicates they are encrypted? If so, as noted, consider that it might be any computer, Apple ID, Wi-Fi password, or device PIN that you've used in the past, working from the assumption that you might have reused a common password that you were using when encryption was first activated. See About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support for details.

Having a Microsoft Exchange email account on your device will force iTunes to make encrypted backups. There could be other routes that enforce this policy setting.

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See Disable iOS backup encryption - Apple Community. Perhaps one of the suggestions there might help.

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