Earlier quoted context omitted.
Ahhh. I miss the days of the username and password being on the same page! I refuse to believe that the average internet user gets confused by a two field form that they need to break it into two (usually slow) steps. Re-architected in (usually flaky) javascript. That takes up (always more) of my time! (sorry for the !!!'s. but this one really gets my goat)
That's to support single sign-on. It checks whether to show the in-house sign-in page or to redirect.
We already have technologies such as Kerberos that are supported in every browser and seem like they would solve this problem.
In any case, as a website operator you can mitigate this. Have separate pages for SSO/non-SSO, dynamically hide the password field if the username is associated with an SSO provider, or just ignore the password field and have a subtitle along the lines of "leave password empty for SSO accounts".