What determines retreat path?

[Steel God]

Rule No. 1: Retreat into a hex that is not in an enemy ZOC.

Rule No. 2: Retreat into a hex that has enough stacking points to allow it to enter.

Beyond that, it's a mystery.
 

There are many things that are undocumented and therefore unverifiable, but which I categorically swear exist somewhere buried in the code of the engine.  For example, I have very good results using suppressive fire, shelling a target and then moving adjacent to it even in T mode will not trigger OpFire (not counting having bled off it's 3 shots before hand).  Likewise with retreat paths from assaults, not always but fairly reliably, if all other things are equal, I can usually count on a retreated unit going directly back from the direction of the assault.  I've used that tactic to assault in an X pattern and thereby open a hole in a line if it's not defended in depth. 

Now I freely admit that I can't prove any of that, and it may be completely in my imagination, but it works for me and I've had my fair share of success, so I'm gonna keep on keeping on with it.