On the Voidwalker there is a toggle to autocast Threatening Presence with a right click. But a left click changes the icon and that can also be set to autocast. What is that all about?
Left click activates/deactivates the ability like a light switch. The purple icon is the inactivated ability. The white icon means it is activated.
Right click (glowing rotating border) tells the pet to activate the ability when it is in combat, if the ability is not yet activated.
Generally just make sure the icon has a glowing rotating border with right click for most content.