I checked my files (community guide etc.) and although I did find MOVEFORE, MOVETOTARGET and MOVEUP I couldn't find a MOVEBACK action. I also found that the CHOOSESTRAFE action may further choose only left, right and forward.
Which makes me think that the engine doesn't allow NPCs to move backwards yet.
To have an NPC runaway from the player, scripts can combine the following
any condition (like distance/health/damage/etc)
ROTATOPLAYER
ROTATEIY=180
RUNFORE=x
There is also a FORCEBACK=x command thats usually used when p.e. a weapon like a shotgun is fired at an NPC.
Hope that helped a bit
In case you find my grammar and spelling weird ---> native German speaker ^^