If you back up, make sure to back up to something (like an external drive or flash drive) that stays disconnected from the computer most of the time. If you're backing up to a network drive or to some cloud services, it may have the ability to encrypt your backups as well, since it has access to them.

Better yet, use TWO flash drives in rotation, so if it happens to start encrypting while you're backing up, the backup on the other drive that's not connected is safe.