You'd have thought that the cat would want to stay on four legs in a confrontation situation, I mean, if any of mine are relaxing and they hear a frightening noise, the first thing that they do is to stand up.

I always assumed that this was done to be in a better starting position should a fight occur or to look bigger. They make their tales go thick to look more threatening, so it's a given that they should stand up first really!

However, this is not always the case:

http://cats.lovetoknow.com/Cat_Behavior

It's when they are using their back leg claws to inflict injury on the belly of their opponent. That's what it's all about.

I wonder if any other animals do this? I doubt that dogs do with their claws being so blunt.