This is just the way it is. If you add a hierarchy, its first level below the "All" member is always expanded. Of course, you can remove and add the hierarchy on demand, but if you add it, it will always be expanded in Excel pivot tables - as well as in most other client tools.
To get the behavior that you want, you could create a hierarchy (in whichever dimension you feel it would make sense, maybe a new one) that contains the levels of the date hierarchy, and below that a level containing the document number. But this would mean that you would have to replicate the date dimensions' attributes to the dimension containing this hierarchy.