UINavigationController and low-memory warnings

Question

I use a UINavigationController (included in my app's main NIB file) with the following view-controller structure:

navigationController
        |
        V
      RootVC  --->  LevelOneVC  --->  LevelTwoVC

All my view controllers' views are loaded from NIB files using the same name as the view controller classes (RootVC.xib, etc).

The problem: When I push a LevelOneVC, then a LevelTwoVC onto the navigation stack, and then simulate/get a low-memory warning, the following happens: After popping the upper-most view controller (LevelTwoVC), when trying to pop the subsequently-current view controller (LevelOneVC), the Back button flies out to the right, but the RootVC's view does not appear.

It all looks as if, for the navigation controller, the RootVC is indeed now the current controller, but that "only" its view could not be re-loaded from the NIB file.

Any ideas as to why this might be happening? Thanks!