When both groups were dispatched by their masters in search of the Hobbits, Saruman instructed his Uruk-hai to bring them to Isengard instead of taking them to Sauron in Mordor. When they got to Rohan, the Mordor Orcs wanted to carry on the south and then east to Mordor, but the Isengard Orcs insisted on taking the Hobbits west to Saruman. This misunderstanding started a fight.