Leverage vs. reach. It is easier to control your opponents weapon if you manipulate the leaver point thats farthest away from the wrist. Given that he’s facing a gladus and buckler (equivailant to a sidearm when compared to a lance or javelin), he does not need to worry about reach, and given that he can’t two-hand the weapon without disusing the net, his best option is to give his opponent a shorter fulcrum while still retaining enough length to control the distance.
It’s also worth noting that they’re not like… Just trying to kill each other. They’re putting on a show. Sure he could probably just impale his opponent, but this was less deathmatch and more WWE. They had sponsors, insurance, and lived long lives under the less bloodthirsty emperors, becoming celebrities throughout the empire.