Accounts named twitter_

I have some users named twitter_<user name> with email addresses like twitter.<user name>@example.com. As one of the user names concerned is myself, I am quite sure that these accounts were not created by any users. Unlike the spurious spamming or hacking accounts, I never receive an email, in my role as administrator of the site, that the account was created. I suppose that they were created by a plugin that was not configured correctly or has a bug. But I have no idea which. I have looked through the source code and through the database, but cannot find what might be at the source of these accounts. Can anyone think of how these accounts came to be created?