EditorFor doesn't accept HTML attributes as a parameter. The parameter you are using is actually additionalViewData
Use TextBoxFor instead
@Html.TextBoxFor(m => m.UserName, new { @class = "form-control" })
Edit
As of MVC 5.1, HTML attributes can be passed to the EditorTemplate using this syntax
@Html.EditorFor(m => m.UserName, new { htmlAttributes = new { @class = "form-control" } })