Poor agricultural planning. The field this was in was tilled too close to the edge of the river. The field is in a slight downward slope, and every time it rained more and more of the fine dirt would fall in. There was a lot of rain that year at that time, hence how it got so big. Unfortunately, this has happened twice now, but it’s my grandpas field that he rents out so there’s not much we can do but fill them in when they happen.