Calling arc patch on a diff that's dependent on a different diff tries to patch the parent first.
To patch the parent a child workflow is created, and a conduit is passed down, but the credentials are not and it is not marked as authenticated.
Then when the child tries to get the commit message for the dependency, it checks isConduitAuthenticated() https://secure.phabricator.com/diffusion/ARC/browse/master/src/workflow/ArcanistPatchWorkflow.php;2c3268f03ed70d3221eb1642bIc99ebb39b12902e$800 and on failure pops up an interactive editor for the commit message.
Instead we just pass down the credentials to the childred and mark them as authenticated, so this is not a problem.