Based upon the subsequent signal that U2 gave, indicating a circular motion around the ground, I'm guessing that perhaps he felt that the runner should have paid attention to where the ball was located and make some minor effort to adjust his position on the bag to give the fielder a chance to field the ball. After all, the runner simply stood on the bag with his back to the fielder, making no effort to watch what was going on. Perhaps U2 felt that the runner's indifference was enough justification to rule intent.
But again, I'm just guessing. The rule, as written, doesn't require the runner to make an active effort to avoid hindering the fielder while staying in contact with the base.
That said, he obviously kicked the penalty, since there was only one out at the time. He should have ruled both the runner and the batter-runner out.