[dllthomas]

Imagine X and Y axes, perpendicular as usual. If you become 3 units more X, that doesn't change how Y you are. Contrariwise, if we nudge our axes out of orthogonality, now moving along the X axes changes where I am on the Y. Obviously the use here is metaphorical, but that's the sense meant.

Edited to add: consider "independence" and orthogonality in vector spaces, if you want to get mathematically precise about it.