» Fri Jun 22, 2012 1:38 pm
Simple answer is sometimes cells don't render just quite right. We have seen instances in the past where entire cities did not get rendered and all the way down to slaughterfish spawning on land. Good news is, the next time the cell renders, it is OK. Note, this is why saving as soon as you enter a cell can lead to issues down the road, not the missing things, but the cell needs a bit of time to render and settle down.
These animals are designated to be rendered in the cell when it loads. Since the game can put them just about anywhere in the cell at render, which could be above a place that is above the slope or the ground, and that is why they fall to the ground. It is rare, but don't worry about it when it happens.
If you need an in-game reason for it, just say a dragon from the next valley over threw it at you.