What determines retreat path?

[Steel God]

 Nice to know there's still some mystery left in the world.

YES - THAT!!  

I'm as guilty as the next person of wanting to understand the rules of the game so I can maximize my effectiveness, but I would definitely say I am much more willing than the average player to pass off those things which I don't understand or can't quite make sense of as "the friction of war" and move on.  I've encountered some poor souls who simply can not enjoy a game if they can't figure out why their assault failed or what have you.