My best guess (which may not even be remotely close to what the problem actually is) would be oversized bounding box or something went a bit awry with the physics if you're using physics XD

If they were physically placed there to find it's possible they were just not placed quite on the ground, I've done that plenty of times with characters and not noticed they were walking sligthtly above the ground til render time and the shadows gave it away and then I'm all ... because I have to rerender XD