A site navigation bar automatically links to all pages and appears on all pages within a document.
Adding a new page will cause the site navigation to be copied to the new page, and a new button added to the bar bearing the button name as applied to the new page. Moving the site navigation bar on one page will automatically cause its position to be updated on subsequent pages within the document.
A site navigation bar will only apply new pages as top level buttons. Its currently not possible to automatically associate a new page with a sub menu item.
To prevent this behaviour, double click your Navbar to open the navigation bar properties and remove the check from the checkbox 'Site navigation bar'.