Arnold was expecting Tiamut to engage first. However, to his surprise, Tiamut decided to stay back while Talos charged at him. Talos was usually the calm one and tried to gain as much information about the enemy as he could before engaging. That was the reason, Talos rarely ever actively engaged in a battle unless he absolutely had to.
Maybe they knew Arnold would be expecting something like that, thus they decided to change the script and let Talos attack while Tiamut backed him up. Talos did not waste any time and aimed at Arnold's head. In the following moment, Arnold found himself in the middle of a barrage of bullets. But to everyone's surprise, he didn't even bother to dodge the bullets.
