Any tiny yaw or pitch change was very, very exaggerated by the long distance from the windshield to the eye, and the very small angle of vision. Passengers would weep in terror as the pilots seemed to lose control and the aircraft appeared to swing wildly about the sky.
I'm sure someone can explain the geometry of this illusion in scientific terms that are beyond me.
I think this is related to parallax.
Parallax - Wikipedia, the free encyclopedia
Hope this helps.