Would be interesting to know what third party service it was and how they were able to make that link. Dropbox uses email address + password for authentication

Would be interesting to know what third party service it was and how they were able to make that link. Dropbox uses email address + password for authentication. If you have a list of email addresses and passwords from accounts on some other compromised service, why not try them against Dropbox to see if any of those people have Dropbox accounts with the same password?