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.

Removing Residual Styling Support Guide Residual Styling Troubleshooting Guide