Shift tab and drop down menus

Shift tab and drop down menus

I created a global navigation menu with drop downs.  I tinkered with it to allow keyboard users to access the menu.  It's tedious for the user, but it works.  However, if the user uses Shift+ tab to tab backwards to the previous top menu list item, the drop down menus persist instead of "disappearing".  The test site can be seen here:  http://checkoutacollege.com:8080/.  Can anyone tell me why the menus persist and how I address this?