One of my dwarfs is bringing the corpses of dead animals into my dining for another dwarf to carry to the refuse pile. Why is that happening? Is it affecting my fortress in a negative way?
As Raven Dreamer mentioned, cats love to catch vermin and drop their corpses around the fortress for your dwarfs to clean up. This is probably the culprit.
Otherwise, I believe a dwarf may drop his load if his hauling job is cancelled (e.g. by encountering an enemy or being unable to reach the destination).