Could it be some malware? I remember some phishing attempt recently with an apple iCloud prompt. Have to search to find the story. Couldn't find at first try. Will update here once found.
Are you sure it wasn't asking you to log in to your iCloud account if you where using iCloud to share your password data?
Yes, but with BBEdit, not 1Password. It happened tonight.
I got same for 'Download Shuttle'
Today, this happened to me, too. Very weird and very annoying indeed.
Authenticating to a service such as the App Store via password without using a password manager feels very anachronistic and insecure.
This always makes me think that there has to be a better solution than passwords for authentication. Password managers such as 1Password are great but they still are a work-around for an essentially broken system. Plus as long as there's no close OS integration we'll always run into situations like this.