When the flocks were brought back to the water, the men who had originally scattered the flocks came to do so again. They would have had a similar problem in recovering any of the flock, and apparently Ammon was able to recover the majority. Bereft of their plunder, the men attempted to recover their prize.
One of the questions that remains unanswered is why no one had attempted physical force as a solution prior to Ammon. Since Ammon was a servant, as were the others, we must assume that the others would be just as likely as Ammon to be armed. Perhaps the difference lay in the training with the arms. There was some situation in play that appears to have assured that the men who did the scattering would be considered more powerful than the defendants.
Ammon has the men he is with circle the flocks. This is to prevent another flight of the flock. Ammon was likely certain that he would be at the center of a disturbance, and it had been some type of disturbance that had frightened and scattered the flocks before. Therefore he takes this precautionary measure.