IIS requiring user to log in twice

I've been noticing some strange behavior on an asp.net application that uses windows authentication to secure an admin directory. Basic auth is disabled.

For most pages within that directory everything works as expected, but some pages pop up a second credentials box. The strange this is that when the exact same credentials are typed in the request is completed as normal.

What I don't understand is why the cached credentials work on some pages but not on others. Any ideas as to how to begin debugging? The server is 2008R2.