Well, Qt has both a QMenu and a QMenuItem. Only the QMenuItem is supposed to trigger actions. The QMenu itself only has QMenuItems and submenus. So, its action is already defined.
Usually, the QMenuItems have QActions associated with it. For the QMenu::triggered signal the documentation states:
This signal is emitted when an action in this menu is triggered.
To me this reads that you have several entries inside the QMenu and one of these entries triggers an action. This is also why this signal contains the QAction as a parameter. My understanding would mean that clicking on the QMenu itself does not emit a triggered signal, but only clicking on a QMenuItem does.