If you would like the current submenu to be open by default on page load, you can do this in the Menu Item Settings.
On the top level item (the parent of the submenu you wish to remain open), open the Menu Item Settings and click the Submenu tab on the left. Check the Show submenu when current setting. Then save your changes.
The submenu will be open by default when a page in the submenu (or the item itself) is loaded. The menu will close on hover-out (for the hover triggers) or click-off, or on activation of another submenu.
Please note if your submenu is opening on the current item page, and you haven’t enabled this setting, it is likely that your theme is interfering – please see the Troubleshooter: Current submenu open by default > Theme replaces current menu item classes with ‘active’ class