This means that your theme’s menu styles are affecting UberMenu adversely. Usually, the easiest fix is to simply change the ID or class of the containing div (the one surrounding #megaMenu, or in your code the one surrounding wp_nav_menu()). Otherwise, you can delete the menu styles from your theme’s CSS. A third option is to counteract the styles using custom CSS.
Please refer to the following guides on troubleshooting and resolving this issue with your theme.