It was more the fact that this bizarre "error" caused the death of other pilots. That was the shock to Tanikaze. I suppose if you look at it from the viewpoint of Hoshijiro voluntarily sacrificing herself for Tanikaze then the main fault lies with him...

This issue's a little hard for me to work out since while they're pilots they don't seem to have the mental conditioning of soldiers, so whose fault is it that one breaks down in battle like that?