Well, this is just a behaviour of your custom style, I guess.
Your style seems to have a dual behaviour when it comes to navigation tab menus. You can hover over them, and the menu opens. Clicking on the tab also makes the menu open. This applies to all of your tabs.
Obviously, this behaviour is different to how XF behaves by default where hovering over them triggers the menu, and a click always clicks the link.
This might be a behaviour you can change (for all your tabs) in your style's settings.
Assuming that you just don't want the menu at all, it can only be removed using a code edit.