I wrote a article about modelbehaviours a few months ago. You might want to read it at
www.thefuture.dk as it relates to your problem.
I have always thought that the engine checked if the origin of the model was visible. Many mappers have had problems with models not showing up from certain angles. Its cool to know it can be fixed by attaching more of the mesh to bones.
Daano and Zyndrone had lots of problems with in Sandstorm and fixed it by lowering the model in the editor and therefore raising the origin.