Gul'dan was a very decisive Orc.
This decisiveness constantly manifested itself during pivotal moments in his life.
When Gul'dan realized he was not cut out to be a Shaman, he decisively joined the Burning Legion.
Later, when Medivh sought him out to open the Dark Portal, Gul'dan, upon realizing he could not defeat this human Mage, acknowledged his inferiority and encouraged the Orcs to build the Dark Portal with all his might.
Now, realizing that Orgrim held him in low regard and having learned the approximate location of the Tomb of Sargeras, Gul'dan took a moment to think and then decided to make his escape.
Of course, he couldn't just run away blindly.
To find the Tomb of Sargeras and obtain the Divine Artifacts and mysterious powers buried within, he knew he needed enough minions (cannon fodder) to face the terrifying foes that lurked within the Tomb.
So, having resolved to flee, the first thing he did was gather his forces as much as possible to accompany him.
