There are a few things, that might have caused that. If a Sim is away from his or her lot, and is eating or drinking something, they might just set the glass or the plate on the ground, when they are done. If they pick up the plate or glass, then it becomes a trash bag, which they will take to a public trash can, if they can reach it. Another thing that might cause it, is if a rummager rummages through the trash, but doesn't do anything with the pile. Another Sim might come along and pick the pile up & try to put it in a trash can. Sometimes, for whatever reason, a Sim can't seem to reach a trash can, and will drop the pile. It may then get passed around, as Sims pick up the pile, carry it a short distance, then drop it.