When a page is resized to make the hamburger icon appear. The menu elements appear below the menu to overlay any content which then has a fade-out transition to make them disappear.
I’ve seen that, but it’s not something that users will normally see - unless they make a habit of resizing their windows in order to watch hamburger menus appearing and disappearing - so I don’t think it matters.
If you turn off the Animate checkbox it doesn’t happen, but then you lose the transition effect when opening and closing a menu.