Board Thread:Skyrim/@comment-108.220.169.79-20150530165618/@comment-6881629-20150530171917

Area resets will clean up bodys after a few days but some do seem to vanish quicker there might be some limit to the number that can be in one place and that speeds it up especially in busy towns (even Riverwood) to help with game performance.

It could possibly have been also triggered from all the area loading that have happened from going to Whiterun paying the fine and then going back again to Riverwood (all just theorys though).