I noticed that, while harvesting wheat, some of my hoed squares were going back to normal dirt. I know for a fact that the water is spaced correctly, as there had been a point where every single square was moistened. Something occurred to me, and I pressed "Q" a few times. Lo and behold, an item falling onto a square had a chance of destroying it. I know that happens with jumping/falling, but I didn't think items could affect it.