While Nero was fighting against four teams on his own - a single cadet against twenty others - and winning as if that was the most reasonable thing to do, Arin had to fly back to the fort because he heard the sounds of fighting.
As it turned out, one of the teams specialised in stealth. Throughout the match, they hadn't fought a single fight, instead hiding and letting the enemies finish each other off. It was only now that it seemed like the exercise was about to end that they made their move.
Those left behind in the fort were naturally concerned about Nero, but they had been told to protect the fort. With Nero and Arin out, there were just three of them left to hold it, which is why they couldn't afford any mistakes. They maintained their vigilance and watched all sides of the fort for any potential attacks.
