It's happened all four times I've killed one, so I suspect it's the creature specifically.
With three of them, when they were were killed, their corpses were quickly dragged away to another point by the game. The other Kagouti fell through and then got stuck in the bottom of a bridge, and ended up being stretched between the bridge and the ground of the cayon below.

#1
Similar has happened to me in Vanilla Oblivion aswell. NPC and Monsters are sometimes dragged to otherside of Cyrodiil or they are just launched to atmosphere when killed. I dont think its Morroblivion bug, morelike Oblivions system bug.