Weird navigation bar animation while transitioning view controllers

Question

When calling -[UIViewController transitionFromViewController:toViewController:duration:options:animations:completion:], I get a weird navigation bar animation. I've definitely seen that kind of thing before, where a view is added to the screen during an animation, and because it doesn't have a frame to begin with animates from 0,0. But I'm not sure what could be causing it in the navigation bar.

Code used to transition the view controllers:

- (void)setCurrentViewController:(UIViewController<TCSearchResultsViewController> *)currentViewController animated:(BOOL)animated
{
    [self.searchBar resignFirstResponder];

    if (_currentViewController != currentViewController) {
        if (_transitioning) {
            __weak __typeof(self)self_weak = self;
            _afterTransition = ^{
                [self_weak setCurrentViewController:currentViewController animated:animated];
            };
            return;
        }

        UIViewController<TCSearchResultsViewController> *oldViewController = _currentViewController;
        _currentViewController = currentViewController;

        [oldViewController willMoveToParentViewController:nil];
        [self addChildViewController:_currentViewController];

        _currentViewController.view.frame = self.resultsViewContainer.bounds;
        _currentViewController.view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;

        _currentViewController.groups = _searchManager.allObjects;
        _currentViewController.location = _location;

        if (oldViewController != nil) {
            _transitioning = YES;

            [self transitionFromViewController:oldViewController
                              toViewController:_currentViewController
                                      duration:(animated ? 0.5 : 0.0)
                                       options:UIViewAnimationOptionTransitionFlipFromLeft
                                    animations:nil completion:^(BOOL finished) {
                                        [_currentViewController didMoveToParentViewController:self];

                                        // this will make sure UITableViewController catches up to the keyboard position
                                        UIView *firstResponder = self.view.window.firstResponder;
                                        if (firstResponder != nil) {
                                            [firstResponder resignFirstResponder];
                                            [firstResponder becomeFirstResponder];
                                        }

                                        _transitioning = NO;
                                        if (_afterTransition != nil) {
                                            _afterTransition();
                                            _afterTransition = nil;
                                        }
                                    }];
        } else {
            [self addViewController:self.listViewController toView:self.resultsViewContainer];

            _transitioning = NO;
            if (_afterTransition != nil) {
                _afterTransition();
                _afterTransition = nil;
            }
        }
    }
}

Here is a video of it happening: http://f.cl.ly/items/2s0H0n3g3i0C1K470V2V/Nav%20Bar%20Animation.mov

The view controller that is being transitioned is just below the double lines in the video.