Super-Cool Sabu was insulted by the straight jab aimed at him. He was a SWORDSMAN. That was like the easiest maneuver to parry, especially when parrying flesh. That short-sword was a simple swing away from blocking the Rasengan. So here's what happened:
Sabu swung the sword diagonally with his right hand towards the Rasengan arm as it came at him, using the flat side of the blade to deflect Koji's arm to Sabu's right. He didn't want to cut off Koji's arm then take the Rasengan to the face as it flew towards him. Sabu couldn't just halt his momentum, but he could use it. As the cold metal would push Koji's arm out of the way, Sabu would just angle the point towards the Raikage's chest and keep on keeping on, likely running him through with it.
This was a very basic parry Sabu had created, called the Double-Point Reverse. When running at someone, to counter a basic stab was pretty crucial for any swordsman, especially Sabu who was the king of running hastily & blindly towards things. So this reaction was instinctual, like wincing or shivering. He wouldn't even know what he was doing until he was attempting to put his blade in cutie-pie Koji's spine.