Backup encryptiuon cannot get enabled on its own. You or someone with access to the computer had to enable it at some point.
With that said, if you forgot the password that was used for backups, you can remove the encryption for future backups, but any backups already made wil remain innacessible.
On your iPhone, Settings ➜ General ➜ Transfer or Reset iPhone ➜ Reset ➜ Reset All Settings
Connect your iPhone to the computer find it in Finder, iTunes or the Apple devices App, and make sure Encrypt Backup is not checked in the device summary page.

Then you can create a new unencrypted backup.
click here ➜ About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support