It truly looks like plowing. People who see it think that he intended for this. Explanation #2: It is considered like truly working the land. Even though he does not intend for this, it is Melachah. Implied question: R. Shimon permits Davar she'Eino Miskaven! Answer: We say that R. Shimon agrees about a Pesik Reishei (an inevitable consequence, e.g. one cannot cut off a chicken's head) and it will not die; Rabanan (who permit) hold that since he does it through a Shinuy, it is not a Melachah, and it is permitted, for it is not considered plowing.

There is no concern for this (lest it looks like he plows), for one who sees the ditch sees the manure in the ditch.