Secondary menus on responsive pages

Hi,

I’ve set up a sub-menu on this page: http://dev.archaia.yale.edu/about. Upon resizing, the sub menu relocate to the bottom of the page. This page (https://physics.yale.edu/academics), in comparison, has the sub-menu collapsing/hamburger below certain screen sizes, and stay on top of the page. Can you please tell me how to achieve the same?