Why can I not place Master and Detail view next to each other in UISplitViewController on the first run, but upon rotation it works?

I have a split view controller that has a list of items on the left and a detail view on the right. Relevant code in AppDelegate:

let splitViewController = mainView.instantiateViewControllerWithIdentifier("initial") as! UISplitViewController



        let rightNavController = splitViewController.viewControllers.last as! UINavigationController
        let detailViewController = rightNavController.topViewController as! DetailsIpad

        let leftNavController = splitViewController.viewControllers.first as! UINavigationController
        let masterViewController = leftNavController.topViewController as! MainViewController

        masterSplitViewController = masterViewController
        detailSplitViewController = detailViewController

        // Override point for customization after application launch.
        let navigationController = splitViewController.viewControllers[splitViewController.viewControllers.count-1] as! UINavigationController
        navigationController.topViewController!.navigationItem.leftBarButtonItem = splitViewController.displayModeButtonItem()
        splitViewController.delegate = self

        self.window!.rootViewController = splitViewController

When I first launch the app I see that the right part of the split screen takes up all of the screen:

If I rotate the screen, it becomes properly set (probably because both views are present on the screen):

When I set breakpoints everywhere, I see that the detail view on the right gets loaded before the master view on the left (list of items), despite not being called directly. I cannot change the order in which the views of the split screen are called. How can I fix this?