View transition doesn't animate during custom pop segue

Question

I'm using a custom segue to create an expand animation when I push new view controllers onto my navigation stack. If no sourceRect is specified when prepareForSegue:sender: is called, then the destination view controller is expanded from the center of the source view controller.

ExpandSegue.m:

- (id)initWithIdentifier:(NSString *)identifier source:(UIViewController *)source destination:(UIViewController *)destination {
    self = [super initWithIdentifier:identifier source:source destination:destination];

    self.sourceRect = CGRectMake(source.view.center.x, source.view.center.y, 0.0, 0.0);

    return self;
}

- (void)perform {
    UIViewController *sourceViewController = (UIViewController *)self.sourceViewController;
    UIViewController *destinationViewController = (UIViewController *)self.destinationViewController;

    CGRect destinationRect = sourceViewController.view.frame;
    CGFloat tx = self.sourceRect.origin.x + self.sourceRect.size.width/2 - destinationRect.origin.x - destinationRect.size.width/2;
    CGFloat ty = self.sourceRect.origin.y + self.sourceRect.size.height/2 - destinationRect.origin.y - destinationRect.size.height/2;
    CGFloat sx = self.sourceRect.size.width/destinationRect.size.width;
    CGFloat sy = self.sourceRect.size.height/destinationRect.size.height;

    [sourceViewController.view addSubview:destinationViewController.view];
    [destinationViewController.view setFrame:sourceViewController.view.frame];
    [destinationViewController.view setTransform:CGAffineTransformConcat(CGAffineTransformMakeScale(sx, sy), CGAffineTransformMakeTranslation(tx, ty))];

    [UIView animateWithDuration:0.5
                          delay:0.0
                        options:UIViewAnimationCurveEaseOut
                     animations:^{
                         [destinationViewController.view setTransform:CGAffineTransformIdentity];
                     } completion:^(BOOL finished) {
                         [destinationViewController.view removeFromSuperview];
                         [sourceViewController.navigationController pushViewController:destinationViewController animated:NO];
                     }];
}

The expand segue works as expected. I tried to create a collapse segue by reversing the logic from the expand segue, but this has given me problems. I want the source view controller to shrink down to the size of the destinationRect. If no destinationRect is supplied, then I want the view to shrink to the center of the destination view controller's view.

CollapseSegue.m:

- (id)initWithIdentifier:(NSString *)identifier source:(UIViewController *)source destination:(UIViewController *)destination {
    self = [super initWithIdentifier:identifier source:source destination:destination];

    self.destinationRect = CGRectMake(source.view.center.x, source.view.center.y, 0.0, 0.0);

    return self;
}

- (void)perform {
    UIViewController *sourceViewController = (UIViewController *)self.sourceViewController;
    UIViewController *destinationViewController = (UIViewController *)self.destinationViewController;

    CGRect sourceRect = sourceViewController.view.frame;
    CGFloat tx = self.destinationRect.origin.x + self.destinationRect.size.width/2 - sourceRect.origin.x - sourceRect.size.width/2;
    CGFloat ty = self.destinationRect.origin.y + self.destinationRect.size.height/2 - sourceRect.origin.y - sourceRect.size.height/2;
    CGFloat sx = self.destinationRect.size.width/sourceRect.size.width;
    CGFloat sy = self.destinationRect.size.height/sourceRect.size.height;

    [sourceViewController.navigationController popViewControllerAnimated:NO];
    [destinationViewController.view addSubview:sourceViewController.view];
    [sourceViewController.view setFrame:destinationViewController.view.frame];

    [UIView animateWithDuration:0.5
                          delay:0.0
                        options:UIViewAnimationCurveEaseOut
                     animations:^{
                         [sourceViewController.view setTransform:CGAffineTransformConcat(CGAffineTransformMakeScale(sx, sy), CGAffineTransformMakeTranslation(tx, ty))];
                     } completion:^(BOOL finished) {
                         [sourceViewController.view removeFromSuperview];
                     }];
}

Rather than animating nicely, the view for the sourceViewController simply disappears as if I had only called [sourceViewController.navigationController popViewControllerAnimated:NO]; This would seem to indicate that I'm not setting up my view hierarchy properly, but I can't seem to figure out where I'm going wrong! Any help would be greatly appreciated.